JAMB Chemistry

A: A decrease in pressure by expansion

The product side has two moles of gas for each mole of N₂O₄. Decreasing pressure by expansion at fixed temperature favours the side with more gas molecules, producing more NO₂.

C: +7

In neutral HClO₄, hydrogen is +1 and four oxygen atoms total −8. Therefore 1 + x − 8 = 0, so chlorine is +7.

C: HI

At 298.15 K, standard molar gas entropies are approximately HF 173.8, HCl 186.9, HBr 198.7 and HI 206.6 J mol⁻¹ K⁻¹. HI has the largest value in this series.

A: 54.1 g

For Ag⁺ + e⁻ → Ag, one mole of electrons deposits one mole of silver. Q = It = 10 × 4830 = 48,300 C. Mass = 48,300/96,500 × 108 = 54.055 g, or 54.1 g to one decimal place.

B: H₂(g) + I₂(g) ⇌ 2HI(g)

H₂ + I₂ ⇌ 2HI has two moles of gas on each side. Compression at fixed temperature scales numerator and denominator of its pressure quotient equally, so it causes no equilibrium shift in the ideal-gas approximation.

C: Reduced

Zinc is +2 in ZnO and 0 in elemental Zn. The decrease in oxidation number means zinc is reduced; carbon monoxide is oxidised to carbon dioxide.

D: 448 cm³

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O. The carbonate molar mass is 100 g mol⁻¹, so 2 g gives 0.020 mol CO₂. Volume = 0.020 × 22.4 = 0.448 dm³ = 448 cm³.

A: Sublimation

Sublimation is the direct change from solid to gas without passing through the liquid state. Evaporation begins with a liquid, while crystallisation forms a solid.

D: 4.0 g

CuO + H₂SO₄ → CuSO₄ + H₂O is a 1:1 reaction. The acid amount is 4.9/98 = 0.050 mol. The required CuO mass is 0.050 × 80 = 4.0 g.

C: CₙH₂ₙ₊₁

An acyclic saturated alkyl group is formed by removing one hydrogen from an alkane, CₙH₂ₙ₊₂. Its formula is therefore CₙH₂ₙ₊₁–.

D: C₂H₄

Concentrated sulfuric acid at about 180 °C dehydrates ethanol: C₂H₅OH → C₂H₄ + H₂O. The product is ethene.

C: Decrease the soap’s solubility

Concentrated sodium chloride lowers the solubility of sodium soaps in the aqueous mixture, allowing soap to separate. This separation is called salting out.

A: Releases a large amount of heat during combustion

Acetylene burns in oxygen with a concentrated, high-temperature flame. The heat can melt the metal locally so a weld can form.

C: 2-Methylbutane

2-Methylbutane has five carbon atoms and molecular formula C₅H₁₂. Butane and 2-methylpropane have only four carbons; the structure labelled 2-ethylbutane has six.

B: Hydrolysis

In the reverse direction, an ester reacts with water to form an alcohol and a carboxylic acid. This is ester hydrolysis. Alkaline hydrolysis producing a carboxylate salt is called saponification.

C: Decarboxylation

The products contain carbon dioxide and a hydrocarbon with one fewer carbon atom than the acid. Removal of the carboxyl group as CO₂ is decarboxylation.

C: High acidity

Acidic conditions often increase dissolution and mobility of metal-containing minerals. This can allow metal ions to leach through soil and contaminate groundwater.

A: Na₂CO₃

A normal salt contains no replaceable acidic hydrogen from incomplete neutralisation. Na₂CO₃ is the fully neutralised sodium salt of carbonic acid; NaHCO₃, NaHSO₄ and NaHS are acid salts. Normal salt does not mean its solution has neutral pH.

D: 2.0 cm³

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. NaOH amount = 0.020 × 0.1 = 0.0020 mol, requiring 0.0010 mol acid. Volume = 0.0010/0.5 = 0.0020 dm³ = 2.0 cm³.

B: Solution

The dissolved portion of calcium sulfate exists as dispersed ions, forming a true solution. Its low solubility limits how much dissolves; it does not make the dissolved portion a colloid.

C: Have weak attractions relative to their kinetic energy

In a dilute gas, intermolecular attractions are weak compared with molecular kinetic energy. The molecules move freely and do not maintain the regular fixed arrangement found in a crystal.

D: Spherical

An s orbital has spherical symmetry: its probability density at a given distance from the nucleus is independent of direction.

C: Neon and hydrogen

Hydrogen is combustible and can react with oxygen supplied by air. Neon is inert and acts as a diluent. The other listed mixtures contain no readily combustible gas.

A: Electronegativity

Chlorine attracts bonding electrons more strongly than hydrogen, making H–Cl polar. In Cl–Cl the identical atoms share equally. The relevant property is electronegativity; molecular HCl is polar covalent.

A: 8 valence electrons

A halogen atom has seven valence electrons. Gaining one electron forms a halide ion with a completed outer-shell octet.

B: −273 °C

Extrapolating the ideal-gas Charles-law line gives zero volume at 0 K, approximately −273 °C. Real gases condense before reaching this extrapolated limit, so it is not a claim that a real gas physically occupies zero volume.

A: Hydrogen and carbon monoxide

Steam reacts with red-hot carbon: C + H₂O(g) → CO + H₂. The mixture of carbon monoxide and hydrogen is called water gas.

D: Mordant

A mordant helps fix a dye to a fibre. Aluminium hydroxide can bind dye molecules and support their attachment to the material.

B: d electrons as well as outer s electrons

The outer s and nearby d electrons of transition metals can participate in bonding or ion formation. Their relatively similar energies allow different numbers of electrons to be involved, producing several oxidation states.

D: Charcoal

Porous charcoal adsorbs coloured substances onto its large internal surface. Activated charcoal is therefore useful for removing colour from solutions such as sugar solutions.

A: Cooling a molten mixture of the metals

A common alloy-making method melts and mixes the constituent metals, then allows the mixture to solidify. This distributes the constituents through the resulting metallic material.

B: Reduction

Sulfur dioxide acts as a reducing bleaching agent, converting susceptible coloured substances into colourless forms. In some materials the colour can return when air reoxidises the reduced substance.

D: 59.26%

The formula contains 2 Al, 3 S, 14 O and 4 H atoms. Its molar mass is 54 + 96 + 224 + 4 = 378 g mol⁻¹. Oxygen contributes 224 g, so its percentage is 224/378 × 100 = 59.26% to two decimal places.

D: 8 and 2

Three copper atoms lose six electrons in total. Two nitrate nitrogen atoms each gain three electrons to form NO; six further nitrate groups remain with copper. Balancing H and O gives 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 4H₂O + 2NO.

A: Octet configuration

Neon and the heavier noble gases have eight outer-shell electrons, a stable octet. Helium is a separate case with its first shell filled by two electrons.

B: Increase

Mean translational kinetic energy of gas particles is proportional to absolute temperature: average Eₖ = 3kT/2. Increasing temperature therefore increases their average kinetic energy.

A: negatively charged

Cathode rays are streams of electrons. Collection of these electrons by the electrometer electrode adds negative charge.

D: Van der Waals attraction

Van der Waals interactions are generally much weaker individually than ordinary ionic or covalent bonds. Coordinate covalent bonds are also covalent bonds.

D: H₃O⁺

Hydronium, H₃O⁺, donates a proton in aqueous acid-base reactions. K⁺ and NO₃⁻ do not appreciably acidify water, while S²⁻ acts as a base.

B: Using suitable aerobic bacteria

Suitable aerobic bacteria break down some petroleum hydrocarbons in the presence of oxygen. This bioremediation requires appropriate conditions and does not instantly remove every oil component.

A: Increasing temperature

The forward reaction is endothermic when ΔH is positive. Raising temperature favours the heat-absorbing direction and shifts equilibrium towards products.

B: Oxidation occurs

Oxidation occurs at the anode: a species loses electrons. An anode may dissolve if it is an active metal, but dissolution is not required for every anode reaction.

A: Activation energy

A catalyst provides a pathway with lower activation energy. It does not alter the energies of starting reactants and final products, so reaction enthalpy is unchanged.

B: Y becomes reduced

An oxidising agent accepts electrons and is reduced. Thus Y is reduced while reducing agent Z loses electrons and is oxidised.

D: H⁺ and Cl⁻

Hydrogen forms at the cathode and chlorine at the anode. Introductory ion-discharge notation describes these as H⁺ and Cl⁻ discharge. The cathode process is more directly written 2H₂O + 2e⁻ → H₂ + 2OH⁻.

B: −42 kJ mol⁻¹

Use products minus reactants: ΔH° = [−394 + 0] − [−110 + (−242)] = −42 kJ mol⁻¹. Hydrogen in its standard elemental state has zero standard enthalpy of formation.

B: Sodium acetate solution

Sodium acetate solution contains mobile Na⁺ and CH₃COO⁻ ions. Solid KOH has ions fixed in a lattice; mercury conducts through electrons rather than electrolytic ion movement.

C: d

Many transition-metal properties arise from incompletely filled d subshells in their atoms or common ions. These states help explain variable oxidation states and many coloured complexes.

C: It is an acid anhydride

SO₂ is an acidic oxide, conventionally described as the anhydride of sulfurous acid. It acidifies water and is pungent rather than odourless. Dissolving it in water does not directly produce sulfuric acid.

B: Cu(NO₃)₂

A little aqueous ammonia produces blue Cu(OH)₂ from Cu²⁺. Excess ammonia dissolves it by forming a deep-blue soluble copper-ammonia complex, commonly written [Cu(NH₃)₄(H₂O)₂]²⁺.

D: Dark blue

Chlorine oxidises iodide to iodine: Cl₂ + 2I⁻ → I₂ + 2Cl⁻. Liberated iodine forms a dark-blue complex with starch, giving the characteristic initial colour.

B: Oxidation

In basic oxygen steelmaking, oxygen oxidises excess carbon and other impurities in molten iron. Carbon leaves mainly as gases, while suitable oxides enter the slag.

B: CH₃CH₂Br

Hydrogen bromide adds across ethene’s double bond: CH₂=CH₂ + HBr → CH₃CH₂Br. Both ethene carbons are equivalent, so no regioselectivity distinction is needed.

C: 1:2:1

Simple monosaccharides such as glucose have the general formula (CH₂O)ₙ, giving C:H:O = 1:2:1. This ratio is not universal for every carbohydrate; polysaccharide formation removes water.

A: C₃H₇COOC₂H₅

Butanoate contributes CH₃CH₂CH₂COO–, written C₃H₇COO–. The ethyl group attached through oxygen is C₂H₅, so ethyl butanoate is C₃H₇COOC₂H₅.

D: Substitution

Benzene characteristically undergoes electrophilic substitution, replacing a ring hydrogen while restoring its aromatic electron system. Addition is possible under more forcing conditions but is not its characteristic reaction.

D: Ethanoic acid

Excess acidified dichromate oxidises ethanol through ethanal to ethanoic acid. Heating under reflux retains volatile material and allows oxidation to the carboxylic acid.

D: Reforming

Catalytic reforming rearranges hydrocarbon structures and can form branched, cyclic or aromatic molecules. It improves fuel properties such as octane rating; cracking chiefly breaks larger molecules into smaller ones.

B: Cellulose

Cotton fibres consist mainly of cellulose, a polymer of glucose units joined by β(1→4) glycosidic bonds. Starch is a different glucose polymer used chiefly for energy storage.

A: Methane

Natural gas consists mainly of methane, CH₄, although its composition varies and it can also contain ethane, propane and other components.

A: Empirical formula

Ethanoic acid has molecular formula C₂H₄O₂. Dividing all subscripts by two gives the simplest whole-number ratio CH₂O, its empirical formula.

A: 7 mol argon

Count moles of atoms: argon gives 7 × 1 = 7; chlorine gives 4 × 2 = 8; ozone gives 3 × 3 = 9; butane gives 1 × (4 + 10) = 14. The argon sample therefore contains the fewest atoms.

B: Move at different speeds in the column

Ink components have different affinities for the stationary phase and the moving solvent. They therefore travel at different rates and separate into bands; chemical reaction between the components is not required.

B: NO

Nitric oxide, NO, binds to iron in the nitrosyl complex responsible for the brown ring. The colour belongs to the complex; free NO gas is colourless.

C: AlCl₃

Adding a limited amount of hydroxide to aluminium chloride forms white Al(OH)₃: Al³⁺ + 3OH⁻ → Al(OH)₃(s). The precipitate dissolves in excess NaOH because aluminium hydroxide is amphoteric.

B: An addition reaction

Hydrogen adds across the alkene C=C bond in the presence of a suitable catalyst. The carbon skeleton remains intact and a saturated alkane forms, so this is an addition reaction.

C: Ethanal

Ethanol first oxidises to ethanal, CH₃CHO. Further oxidation converts the aldehyde group into the carboxylic acid group of ethanoic acid.

B: Secondary alkanol

In CH₃CH₂CH(OH)CH₃, the carbon bearing OH is bonded to two other carbon atoms and one hydrogen. It is therefore a secondary alcohol, butan-2-ol.

C: Saponification

Boiling a fat with aqueous sodium hydroxide breaks its ester bonds to form glycerol and sodium salts of fatty acids, which are soaps. This specific alkaline hydrolysis is called saponification.

D: Na₂CO₃

Soda-lime glass is made using silica, limestone and sodium carbonate. Sodium carbonate supplies sodium oxide in the melt and acts as a flux, lowering the temperature needed to process silica.

C: Carbon

Carbon forms strong covalent bonds with other carbon atoms, supporting extensive chains, branched structures and rings. This extensive catenation is central to organic chemistry.

D: Fermentation after starch hydrolysis

Starch is first hydrolysed into fermentable sugars. Yeast can then convert those sugars into ethanol and carbon dioxide. Distillation separates ethanol after production but does not convert starch into ethanol.

D: Na

Sodium is sufficiently reactive to release hydrogen from aqueous hydrochloric acid, producing sodium chloride. Silver, gold and copper do not displace hydrogen from dilute non-oxidising hydrochloric acid under ordinary conditions.

C: Its mass is approximately 1840 times the mass of an electron

A proton is about 1836 times as massive as an electron, often rounded to 1840 in school calculations. Its mass is about 1.67 × 10⁻²⁴ g. A molar mass describes one mole, not one particle, and nuclei do not all have equal numbers of protons and neutrons.

A: ¹⁴₇N

In beta-minus decay a neutron changes into a proton, so atomic number increases by one while mass number stays the same. Carbon-14, with atomic number 6, becomes nitrogen-14 with atomic number 7.

D: AlCl₃

Al³⁺ has high charge and small size, giving strong polarising power. It distorts the chloride electron cloud, increasing covalent character; AlCl₃ is therefore the least ionic of these choices.

B: Respiration, decay and combustion

Respiration, aerobic decomposition and combustion release carbon dioxide. Photosynthesis instead takes up CO₂, and transpiration chiefly releases water vapour.

B: Atoms of different elements combine in simple whole-number ratios

Atoms combine in whole-number ratios in the formulas of stoichiometric compounds. Atoms have subatomic structure, isotopes of an element need not be identical in mass, and nuclear processes can transform atoms, so those absolute older claims require revision.

B: 0.33 atm

The oxygen mole fraction is 0.66/(0.75 + 0.66) = 0.4681. Dalton’s law gives p(O₂) = 0.4681 × 0.7 = 0.3277 atm, or 0.33 atm.

B: Pale green

Hydrogen sulfide reduces Fe³⁺ to Fe²⁺, whose aqueous solution is pale green: 2Fe³⁺ + H₂S → 2Fe²⁺ + S + 2H⁺. Sulfur also separates as a solid.

C: ΔᵣG = 0

At equilibrium, the reaction Gibbs energy ΔᵣG is zero, so neither direction has a thermodynamic driving force. This does not require the standard reaction Gibbs energy ΔᵣG° to be zero.

C: +1 to 0

Sulfide has oxidation number −2 in Cu₂S, so each copper is +1. Elemental copper has oxidation number 0, giving the change +1 → 0.

B: Hydrogen and oxygen gases

With inert platinum electrodes, water is reduced to hydrogen at the cathode and hydroxide is oxidised to oxygen at the anode. The net change is 2H₂O → 2H₂ + O₂.

A: The yield of PCl₃ increases

The product side has two moles of gas for each mole on the reactant side. Expansion at fixed temperature lowers pressure and favours the products, increasing the equilibrium amount of PCl₃.

B: Activation energy

The Arrhenius equation is k = A exp(−Eₐ/RT). It relates the rate constant to activation energy and temperature, with A the pre-exponential factor. At fixed A and T, a lower activation energy gives a larger rate constant.

C: An exothermic change

Dissolving sodium hydroxide releases heat overall because hydration of its ions releases more energy than is needed to separate the solid lattice. The solution warms, so the change is exothermic.

C: Blue to pink

Anhydrous cobalt(II) chloride is blue. On taking up water it forms hydrated species that appear pink, producing the blue-to-pink change used in a water test.

B: Unsaturated

The amount is 3.40/101 = 0.0337 mol. With negligible volume change, concentration is 0.0337/0.250 = 0.135 mol dm⁻³, below the solubility of 0.34 mol dm⁻³. More salt can dissolve, so the solution is unsaturated.

C: 0.5 mol dm⁻³

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The carbonate amount is 0.025 × 0.2 = 0.005 mol, requiring 0.010 mol HCl. The acid concentration is 0.010/0.020 = 0.50 mol dm⁻³.

B: Efflorescence

Efflorescence is the loss of water of crystallisation from a hydrated salt on exposure to air. Deliquescence instead involves absorbing moisture until the solid dissolves.

B: Greater than 8.4

Adding concentrated sodium hydroxide increases the hydroxide concentration and decreases the hydrogen-ion concentration. The resulting pH is greater than 8.4.

D: Hydrogen bonding

The OH group in a small alcohol such as ethanol can form hydrogen bonds with water. This favours mixing; solubility decreases as the non-polar carbon chain becomes longer.

A: Calcium hydrogencarbonate

On heating, dissolved calcium hydrogencarbonate decomposes: Ca(HCO₃)₂ → CaCO₃(s) + CO₂ + H₂O. Insoluble calcium carbonate coats the kettle as scale.

A: 72.0 g

The equation 2H₂ + O₂ → 2H₂O gives one mole of water per mole of hydrogen. There are 8.0/2 = 4.0 mol H₂, giving 4.0 × 18 = 72.0 g H₂O.

D: 150 °C

At the liquid–vapour phase change, temperature remains at the boiling point while latent heat is supplied. Cooling the vapour at the same pressure causes condensation at that same temperature, 150 °C.

B: Z

Atomic numbers 2, 6, 16 and 20 identify helium, carbon, sulfur and calcium respectively. Calcium, labelled Z, is the metal; the others are non-metals.

B: A covalent bond

Each atom supplies an electron to a shared pair. Sharing an electron pair forms an ordinary covalent bond. A coordinate covalent bond would have both shared electrons supplied initially by one atom.

B: It increases as absolute temperature increases

For an ideal gas, average translational kinetic energy per molecule is 3kT/2. It increases with absolute temperature. Pressure or volume changes alone do not imply a change in that average energy.

D: Charge on the electron

Millikan’s oil-drop experiment measured the elementary electric charge. The charge-to-mass ratio of the electron had been measured earlier using cathode rays.

C: Aluminium sulfate

Aluminium sulfate is a coagulant. In suitable water conditions it forms aluminium hydroxide flocs that gather fine suspended material so it can be removed by settling and filtration.

C: Catching fire

White phosphorus can ignite in air. Storing it under water limits contact with oxygen and prevents ignition; this storage precaution refers to white phosphorus rather than all phosphorus allotropes.

D: Distillation

Distillation vaporises a solvent and then condenses the vapour into a separate receiver. Non-volatile dissolved substances remain behind, allowing recovery of the solvent.

A: Drying

Deliquescent substances absorb enough moisture from air to dissolve in the absorbed water. Suitable examples such as calcium chloride can remove water vapour and serve as drying agents.

D: 6.30 cm³

Alkaline pyrogallol absorbs oxygen. Taking dry air as 21% oxygen by volume, the removed volume is 0.21 × 30.00 = 6.30 cm³, with temperature and pressure unchanged.

C: A reducing agent

Copper changes from oxidation state 0 to +2, losing electrons. It supplies the electrons that reduce nitrogen from +5 in nitrate to +2 in NO, so copper is the reducing agent.

D: Negative

Two gaseous reactants form a crystalline solid. The loss of gas-particle freedom gives a decrease in the system’s entropy, so ΔS is negative.

A: Decrease

In a dilute electrolyte, reducing concentration lowers the number of mobile charge carriers per unit volume, so electrical conductivity decreases at fixed temperature. Molar conductivity is a different quantity and can increase on dilution.

A: Removing some S

Removing gaseous product S lowers the reaction quotient and drives the equilibrium forward, forming more R. Increasing vessel volume favours the side with more gas molecules, which is the reactant side; a catalyst does not change equilibrium yield.

C: 2H⁺ + 2e⁻ → H₂

Reduction is electron gain. Hydrogen ions gain two electrons to form H₂, reducing hydrogen’s oxidation number from +1 to 0. The other half-equations release electrons and represent oxidation.

B: Exothermic

A negative enthalpy change means the products have lower enthalpy than the reactants and heat is released to the surroundings. Such a reaction is exothermic.

B: Hydrolysis

Acid-catalysed hydrolysis breaks peptide bonds by reaction with water. Prolonged heating with acid can break a protein down into its constituent amino acids.

C: Conversion of sugar to alcohol in the presence of yeast

In alcoholic fermentation, yeast enzymes convert sugars such as glucose into ethanol and carbon dioxide under anaerobic conditions: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

D: Cyclohexane

Complete catalytic hydrogenation adds three molecules of H₂ to each benzene molecule: C₆H₆ + 3H₂ → C₆H₁₂. The saturated ring product is cyclohexane.

B: Hydrochloric acid and oxygen

Chlorine initially reacts with water to form HCl and HClO. Sunlight decomposes hypochlorous acid: 2HClO → 2HCl + O₂. The overall products are hydrochloric acid and oxygen.

A: But-1-ene and but-2-ene

But-1-ene and but-2-ene both have molecular formula C₄H₈ but differ in double-bond position. The other pairs have different molecular formulas and are not isomers.

D: Bind rubber chains together

Sulfur forms cross-links between rubber chains. These links restrict chain slippage and improve the useful elastic and mechanical properties of rubber.

A: Alkaline

Sodium reacts with water to give sodium hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂. Hydroxide ions make the resulting solution alkaline.

C: RCHO

Alkanals contain a terminal –CHO group, so their condensed general formula is RCHO. R is an alkyl group, or hydrogen in methanal.

A: Ca

Calcium gives a brick-red flame in the characteristic flame test. Magnesium burns with an intense white light, while sodium gives a strong yellow flame.

D: Ammonia

Ammonia is less dense than air and very soluble in water. It rises into an inverted gas jar and pushes air downwards, so downward displacement of air is suitable.

B: Silicon(IV) oxide

Silica, SiO₂, is a common gangue impurity in iron ore. Calcium oxide from limestone reacts with it to form calcium silicate slag, removing it from the iron.

C: different sizes

A sieve separates particles by size. Smaller particles pass through its openings while larger particles are retained.

C: Clay

Clay commonly contains aluminosilicate minerals. Kaolinite, for example, has formula Al₂Si₂O₅(OH)₄, containing aluminium, silicon, oxygen and hydrogen. The other listed substances do not contain this set of elements.

C: Oxygen

The air supplies 150 × 0.20 = 30 cm³ oxygen. From 2CO + O₂ → 2CO₂, 50 cm³ CO requires 25 cm³ oxygen. Therefore oxygen is in excess by 5 cm³. Nitrogen is not a reactant here.

B: 200 cm³

Boyle’s law gives P₁V₁ = P₂V₂ at constant temperature. Hence V₂ = (1/1.5) × 300 = 200 cm³. Higher pressure gives a smaller volume.

B: 0.579

Dalton’s law gives mole fraction = partial pressure / total pressure. For oxygen this is 452/780 = 0.579 to three decimal places.

D: degree of movement of their particles

Particles in solids mainly vibrate around fixed positions; liquid particles can move past one another; gas particles move freely over much larger distances. Their freedom of movement distinguishes the states.

B: Detect an element

A radioactive isotope has a characteristic half-life. Measuring decay and comparing it with known data can help identify or detect a radionuclide; knowing its half-life does not itself create, split or irradiate atoms.

C: Linear, bent and tetrahedral

CO₂ has two electron-domain directions around carbon and is linear. H₂O has two bonds and two lone pairs around oxygen, giving a bent molecular shape. CH₄ has four bonding domains and is tetrahedral.

D: DDT

DDT is a manufactured organochlorine pesticide. Nitric oxide, carbon monoxide and formaldehyde can be produced by natural processes, whereas DDT contamination originates from its human manufacture and use.

D: Liberation of heat

When gaseous ions become surrounded by water molecules, favourable ion–dipole attractions form and energy is released. Hydration itself is exothermic, although dissolving an entire solid also requires energy to separate its lattice.

C: Conjugate base

HCl donates a proton to water. The species left after that donation, Cl⁻, is the conjugate base of HCl; H₃O⁺ is the conjugate acid of water.

B: 13.40

For fully dissociated HCl, [H⁺] = 0.25 mol dm⁻³. Thus pH = −log₁₀(0.25) = 0.602 and, at 25 °C in the ideal dilute-solution model, pOH = 14.00 − 0.602 = 13.40.

C: 5e⁻

Manganese changes from +7 in MnO₄⁻ to +2, gaining five electrons. The balanced half-equation is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, with equal charge and atoms on both sides.

C: 9.650 × 10⁴ C

The half-equation uses one mole of electrons for half a mole of Zn. Its charge is therefore one Faraday: Q = 1 × 96,500 = 9.650 × 10⁴ C.

D: M = QZ

Faraday’s first law states that deposited mass is proportional to charge. The electrochemical equivalent Z is mass deposited per unit charge, so M = ZQ. Its units give (g C⁻¹) × C = g.

C: A larger exposed surface area

For the same mass, powder exposes a larger surface area to the acid than granules. More reacting surface is available at once, increasing the reaction rate.

B: A horizontal line at a positive rate

A zero-order reaction has rate = k, independent of reactant concentration over the range where that rate law applies. Its rate-versus-concentration plot is therefore horizontal.

D: Gaseous and dissolved species

In the usual concentration or pressure equilibrium expression, gaseous and dissolved species contribute variable terms. Pure solids and pure liquids have effectively constant activities and are absorbed into the equilibrium constant.

C: Sulfur

Sulfur creates cross-links between rubber polymer chains during vulcanisation. These links improve elasticity, strength and resistance to permanent deformation.

A: Carbon(IV) oxide

Dissolved carbon dioxide produces the fizz and carbonic-acid contribution to the sharp taste of carbonated water. Carbon monoxide is not used for carbonation.

D: CO and H₂

Synthesis gas, or syngas, is a mixture principally of carbon monoxide and hydrogen. It is a feedstock for making fuels and chemicals such as methanol.

C: Violet flame

Potassium produces a characteristic lilac or violet flame. Heating excites electrons, and their return to lower energy levels emits characteristic wavelengths of light.

C: Iron

Haematite is iron(III) oxide, Fe₂O₃, and is an important ore used to obtain iron.

C: Zn

Zinc has a less negative standard reduction potential than aluminium, sodium or calcium, so it has the lowest thermodynamic tendency to be oxidised among these metals. Aluminium surface passivation affects reaction rate but does not place it below zinc in the reactivity series.

B: Isoprene

Natural rubber consists mainly of cis-1,4-polyisoprene. Its chain units are derived from isoprene, 2-methylbuta-1,3-diene; polymerisation changes the monomer bonding while retaining the C₅H₈ composition per unit.

B: Ester

CH₃CH₂C(=O)OCH₂CH₃ contains the ester linkage –C(=O)–O– between carbon groups. It is ethyl propanoate, an ester.

B: Secondary alkanols

Oxidation of a secondary alcohol removes the O–H hydrogen and the hydrogen on the carbon bearing OH, producing a ketone carbonyl. For example, propan-2-ol forms propanone.

B: Glucose and fructose

Sucrose is a disaccharide containing one glucose unit and one fructose unit joined by a glycosidic bond. Hydrolysis releases glucose and fructose.

A: 2.78 g

Each solution contains 0.200 × 0.100 = 0.0200 mol solute. Pb²⁺ + 2Cl⁻ → PbCl₂ requires two chloride ions per lead ion, so chloride limits the precipitate to 0.0100 mol. M(PbCl₂) = 207 + 2(35.5) = 278 g mol⁻¹, giving 2.78 g.

B: 22.00

The amount of gas is 0.05600/22.4 = 0.00250 mol. Its molar mass is 0.11/0.00250 = 44 g mol⁻¹. Vapour density relative to hydrogen is half the relative molecular mass, so it is 22.00.

C: Methane

At the same conditions, diffusion through a porous plug is faster for lower molar mass. The masses are propane 44, oxygen 32, methane 16 and ammonia 17 g mol⁻¹. Methane is the lightest and diffuses fastest.

B: Magnesium

Magnesium combines with oxygen when heated in air: 2Mg + O₂ → 2MgO. The oxygen added to the solid increases its mass. This is different from roasting a sulfide, where sulfur can leave as a gas.

A: 91 K

At constant volume for a fixed amount of ideal gas, P/T is constant. The initial temperature is 273 K. Thus T₂ = 273 × (3/9) = 91 K.

D: Fractional crystallisation

The solids have different solubilities at the same temperature. Controlled cooling and successive collection of crystals can separate them into fractions. This process is fractional crystallisation.

B: 1.5 g

The amount of HCl is 0.250 × 0.5 = 0.125 mol. Mg + 2HCl → MgCl₂ + H₂ requires half as many moles of Mg: 0.0625 mol. With Mg = 24, the mass is 0.0625 × 24 = 1.5 g.

D: Steam at 100 °C

The gaseous state has far more possible spatial arrangements than the solid or liquid states. Steam at 100 °C therefore has the greatest positional disorder among the choices. Equal temperature does not imply liquid and vapour have equal entropy.

B: Ionisation energy

First ionisation energy is the energy required to remove one electron from each gaseous neutral atom in one mole, forming gaseous singly charged ions. For sodium, that electron is in the 3s orbital.

C: Using an ion-exchange resin

A suitable ion-exchange resin removes hardness-causing Ca²⁺ and Mg²⁺ ions by replacing them with ions such as Na⁺. Chlorination and aeration do not perform this exchange.

D: Z

Z occupies Group 0, the noble-gas group, and has a complete outer electron shell. It is therefore the least reactive of the labelled elements. W is an alkali metal, X is carbon and Y is a halogen.

A: HF

Fluorine is very small and highly electronegative, making the H–F bond strongly polar and allowing strong intermolecular hydrogen bonding. Hydrogen bonding in NH₃ is weaker; CH₄ and HCl lack the strong conventional N–H, O–H or F–H pattern.

A: 0.30 mol dm⁻³

The HCl concentration is 0.25/0.50 = 0.50 mol dm⁻³. The 15.00 cm³ portion contains 0.00750 mol HCl. Since Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O, the carbonate amount is 0.00375 mol. Dividing by 0.01250 dm³ gives 0.30 mol dm⁻³.

A: V₂O₅ < K₂Cr₂O₇ < KMnO₄

Oxygen is −2 and potassium +1. In V₂O₅, vanadium is +5; in K₂Cr₂O₇, chromium is +6; and in KMnO₄, manganese is +7. The increasing order is therefore V₂O₅, K₂Cr₂O₇, KMnO₄.

D: The mixture becomes reddish-brown

Acidified iodate oxidises iodide to iodine: IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O. Dissolved iodine, including triiodide in excess iodide, gives the reddish-brown colour.

C: 3.0 × 10⁻³ mol dm⁻³

Let the molar solubility be s. Dissociation gives [Cu²⁺] = s and [IO₃⁻] = 2s. Hence Ksp = s(2s)² = 4s³ = 1.08 × 10⁻⁷. Therefore s = ∛(2.70 × 10⁻⁸) = 3.0 × 10⁻³ mol dm⁻³.

A: Degree of disorder and heat content

Entropy describes the number of accessible microscopic arrangements, often introduced as disorder. Enthalpy is H = U + pV; its change equals heat transferred at constant pressure when only pressure–volume work occurs. The introductory pairing is disorder and heat content, respectively.

C: Vanadium(V) oxide

Vanadium(V) oxide is the catalyst used in the Contact process for converting SO₂ to SO₃. It provides a faster reaction pathway without changing the equilibrium composition.

B: 48,250 C

Forming one mole of O₂ requires transfer of four moles of electrons. For 0.125 mol O₂, the charge is 0.125 × 4 × 96,500 = 48,250 C.

B: X/2 g

For the same metal and current efficiency, deposited mass is proportional to charge It. The second charge is (2I × 10)/(I × 40) = 1/2 of the first, so the deposited mass is X/2 g.

B: The reactants have higher enthalpy than the products

The enthalpy change is H(products) − H(reactants). A negative value of −65.7 kJ mol⁻¹ means that the products have lower enthalpy and heat is released. It does not determine the reaction speed.

D: HCl

Hydrogen chloride dissolves to supply chloride ions. These react with silver ions to form white silver chloride: Ag⁺ + Cl⁻ → AgCl(s). Nitric-acid acidification suppresses interference from carbonate and sulfite precipitates.

A: Dissolve in alkalis

All three halogens react with aqueous alkalis. For example, chlorine disproportionates in cold dilute alkali to chloride and hypochlorite. They are not all liquids, and halogen displacement works in the direction of decreasing oxidising strength rather than mutually in both directions.

B: Na₂SO₃

Sodium sulfite reacts with acid to release sulfur dioxide: Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂. Its pungent gas reduces purple permanganate to Mn²⁺. Sodium sulfide instead gives hydrogen sulfide, characterised by a rotten-egg smell.

A: Evolves a lot of heat when burnt

Hydrogen combustion in oxygen releases substantial energy: 2H₂ + O₂ → 2H₂O. The resulting high-temperature flame provides heat for melting metals; low gas density is not the reason.

A: Calcium hydroxide and ammonium chloride

Heating ammonium chloride with calcium hydroxide releases ammonia: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O. Calcium oxide dries the ammonia without reacting with it as an acidic drying agent would.

C: Strong and light

Duralumin is an aluminium alloy that combines low density with greater strength than pure aluminium. That strength-to-weight advantage makes it useful for structures where mass matters.

B: Magnesium and calcium

Magnesium and calcium are highly reactive metals for which electrolysis of molten compounds is an established extraction route. Zinc, copper and lead can be obtained by chemical reduction routes instead.

A: Iron

Concentrated nitric acid can make iron passive by producing a protective surface film that greatly slows further attack. This is the standard iron passivation example.

C: Diethyl ether

At about 140–145 °C with excess ethanol and concentrated sulfuric acid, intermolecular dehydration produces diethyl ether: 2C₂H₅OH → C₂H₅OC₂H₅ + H₂O. Higher temperatures favour ethene formation.

B: 48.0 g

But-1-en-3-yne is CH₂=CH–C≡CH, with formula C₄H₄ and molar mass 52 g mol⁻¹. One C=C bond consumes one Br₂ and one C≡C consumes two Br₂ on full addition. Thus 5.2/52 × 3 × 160 = 48.0 g bromine is required.

D: Pipes

Polyvinyl chloride is a durable thermoplastic widely used for pipes. Its long polymer chains and material properties suit this use.

C: Methyl propanoate

Ethyl ethanoate is CH₃–C(=O)–O–C₂H₅. Swapping the terminal methyl and ethyl groups gives C₂H₅–C(=O)–O–CH₃. The alcohol-derived group is methyl and the acid-derived part has three carbons, so the ester is methyl propanoate.

B: a chemical change

Calcium oxide reacts with water to form calcium hydroxide: CaO + H₂O → Ca(OH)₂. Formation of a new substance makes it a chemical change. The reaction releases heat.

A: 250 cm³

The equation shows one mole, or one gas volume at equal conditions, of propane requires five of oxygen. Therefore 50 cm³ propane needs 5 × 50 = 250 cm³ oxygen.

C: 28.4 cm³

The initial hydrogen partial pressure is 780 − 10 = 770 mmHg. Use absolute temperatures 300 K and 280 K: V₂ = 30 × (770/760) × (280/300) = 28.37 cm³, or 28.4 cm³.

C: 1:2

M(SO₂) = 32 + 2(16) = 64 g mol⁻¹ and M(CH₄) = 12 + 4 = 16 g mol⁻¹. Graham’s law gives R(SO₂)/R(CH₄) = √(16/64) = 1/2. Thus the rate ratio is 1:2.

B: Electrovalent bond

The two outer electrons can be transferred to two chlorine atoms. This produces a metal cation and chloride anions, held together by electrostatic attraction: an ionic, or electrovalent, bond.

C: Energy during electron transitions

An electron moving from a higher to a lower energy level emits a photon with energy equal to the level difference. Hydrogen’s discrete energy levels produce separate spectral lines.

A: Boiling point

Fractional distillation of liquid air separates nitrogen and oxygen because they have different boiling points. The more volatile nitrogen boils off before oxygen.

C: Nitrogen and noble gases

Burning excess phosphorus removes oxygen from the air. Potassium hydroxide absorbs carbon dioxide and other acidic oxides, and concentrated sulfuric acid removes water vapour. Nitrogen and the noble gases remain.

C: Gas is dispersed in liquid

Soap lather is a foam: bubbles of gas are dispersed through a liquid film. Thus its dispersed phase is gas and its continuous phase is liquid.

D: 12.7

Initial amounts are 0.010 mol HCl and 0.020 mol NaOH. After neutralisation, 0.010 mol OH⁻ remains in 0.200 dm³, so [OH⁻] = 0.050 mol dm⁻³. pOH = 1.30 and pH = 14.00 − 1.30 = 12.70.

D: 100 cm³

M(Na₂CO₃·10H₂O) = 106 + 180 = 286 g mol⁻¹. The sample contains 2.86/286 = 0.0100 mol carbonate. H₂SO₄ reacts with carbonate in a 1:1 ratio, so the required volume is 0.0100/0.100 = 0.100 dm³ = 100 cm³.

A: 1.97 g

The charge is Q = It = 15 × 193 = 2895 C, equivalent to 0.0300 mol electrons. Gold(III) needs three electrons per atom, so 0.0100 mol gold is deposited. Its mass is 0.0100 × 197 = 1.97 g.

D: Cu²⁺ is the oxidising agent

Cu²⁺ gains two electrons to form copper, so it is reduced. The substance that accepts electrons and causes the other substance to be oxidised is the oxidising agent.

A: FeCl₂

Iron changes from +2 in FeCl₂ to +3 in FeCl₃. FeCl₂ is therefore oxidised and supplies the electrons that reduce chlorine, making it the reducing agent.

C: +25 kJ

For the backward reaction, the starting energy is the product level, 15 kJ. The peak is 40 kJ, so the backward activation energy is 40 − 15 = 25 kJ.

D: R = k[X]²[Y]⁰

Independence from [Y] means order zero in Y. Doubling [X] multiplies the rate by four, so 2ⁿ = 4 and n = 2. Therefore R = k[X]²[Y]⁰ = k[X]².

A: Favours the reverse reaction

The positive ΔH means the forward reaction absorbs heat. Cooling favours the exothermic reverse direction, shifting the equilibrium towards chlorine and water vapour.

C: Reducing properties

Nitrogen in NH₃ has oxidation number −3 and becomes 0 in N₂ in each equation. Ammonia is oxidised while the other reactant is reduced, demonstrating that ammonia acts as a reducing agent.

B: Sodium nitrite and ammonium chloride

On warming, sodium nitrite and ammonium chloride produce nitrogen: NaNO₂ + NH₄Cl → NaCl + N₂ + 2H₂O. The ammonium and nitrite ions together supply the two nitrogen atoms. Nitrogen is only slightly soluble in water, allowing collection by displacement of water. Nitrate and nitrite are different ions, NO₃⁻ and NO₂⁻ respectively.

D: A brown precipitate forms

Iron(III) ions form insoluble, reddish-brown iron(III) hydroxide: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s). Iron remains in oxidation state +3; this is precipitation, not reduction to iron(II). Sodium and chloride ions remain in solution.

C: Useful metallic properties can be improved by alloying

Alloying can improve properties needed for a use, such as hardness, strength or corrosion resistance. This is why a suitable alloy may be chosen over a pure metal; the improvement depends on the alloy and application.

B: 2-Methylpentan-3-ol

The longest chain containing the OH group has five carbons. The OH group is at carbon 3 whichever end is used, so choose the direction that gives the methyl substituent position 2 rather than 4. The name is 2-methylpentan-3-ol.

B: Polythene

Ethene monomers join by addition polymerisation. Their double bonds open to form the repeating unit –CH₂–CH₂–, giving polyethene, commonly called polythene.

D: Carbon dioxide and water

Complete combustion converts the carbon atoms of a hydrocarbon into carbon dioxide and its hydrogen atoms into water. Excess oxygen supports complete rather than incomplete combustion.

C: Hydrogen and an alkene

The gas that gives a pop with a lighted splint is hydrogen. The oily product decolourises bromine by addition at carbon–carbon unsaturation, consistent with an alkene from cracking. The matching pair is hydrogen and alkene.

C: C₆H₅OH

C₆H₅OH represents phenol, with an OH group attached to a benzene ring. That aromatic ring distinguishes it from the saturated aliphatic compounds listed.

B: Condensation

The diol and dicarboxylic acid form ester links while eliminating small water molecules. Repeated condensation produces the polyester Terylene.

A: Both undergo substitution reactions

Both can undergo substitution under suitable conditions. Benzene undergoes electrophilic substitution, such as nitration, while hexane undergoes radical substitution with halogens under light. Both are liquids at room temperature.

B: 31%

The reaction is 2H₂ + O₂ → 2H₂O. The 12 cm³ of oxygen consumes 24 cm³ of hydrogen and forms 24 cm³ of steam. Hydrogen remaining is 35 − 24 = 11 cm³. At 110 °C the total gas volume is 11 + 24 = 35 cm³, so hydrogen accounts for 11/35 × 100 = 31% to the nearest whole percent.

D: A pure compound

The horizontal section represents freezing at a constant temperature. In the usual cooling-curve interpretation this is characteristic of a pure substance, making a pure compound the best listed description. The graph alone does not identify its bonding or whether it is a hydrated salt.

B: 31.0

Vapour density relative to hydrogen is half the relative molecular mass. Therefore Mᵣ(XH₃) = 2 × 17.0 = 34.0. Subtracting the three hydrogen atoms gives Aᵣ(X) = 34.0 − 3 = 31.0.

C: 21 s

By Graham’s law, rate is inversely proportional to the square root of molar mass. Time also increases with the volume required. Thus t = 7 × (60/30) × √(71/32) = 20.85 s, or 21 s to the nearest second.

C: The heat of vaporisation is drawn from the body

Evaporation requires energy to overcome attractions between liquid molecules. When that energy is supplied by the body, the body loses thermal energy and cools.

C: 1s²2s²2p⁶3s¹ and 1s²2s¹

The configurations in C describe sodium and lithium. Each has one electron in its outermost shell and belongs to Group 1, so they have similar chemical properties. The other pairs have different outer-shell electron counts.

D: Atomic radius

Atomic radius generally decreases from left to right across a period because the increasing effective nuclear charge attracts electrons more strongly. It increases down a group as additional occupied electron shells are added.

C: 2 and 1

P is sodium, which forms P⁺, and Q is oxygen, which forms Q²⁻ in the oxide. Two P⁺ ions balance one Q²⁻ ion, giving P₂Q. Thus x = 2 and y = 1.

B: 16.2

The relative atomic mass is the abundance-weighted average: (16 × 0.90) + (18 × 0.10) = 16.2. The percentages describe the specified sample.

C: 21%

Heated copper removes oxygen as copper(II) oxide. The volume removed is 200 − 158 = 42 cm³. Comparing volumes at the same temperature and pressure gives oxygen percentage = 42/200 × 100 = 21%.

C: 10⁻⁴ mol dm⁻³

At 25 °C, pH + pOH = 14.00. Thus pOH = 4.00 and [OH⁻] = 10⁻⁴ mol dm⁻³.

C: 0.104 mol dm-3

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. The NaOH amount is 0.02500 × 0.125 = 0.003125 mol, so acid amount is half this. Acid concentration = 0.0015625/0.01500 = 0.104 mol dm⁻³.

A: Acidic

Copper(II) ions are reduced to copper at the cathode. At the platinum anode, water is oxidised: 2H₂O → O₂ + 4H⁺ + 4e⁻. The increasing H⁺ concentration makes the solution more acidic.

B: +3

Potassium contributes +1 per atom and chloride −1. For neutral K₃ZCl₆, 3(+1) + Z + 6(−1) = 0, so Z has oxidation number +3.

B: −198 kJ

Reaction enthalpy is the sum of product formation enthalpies minus the reactant sum. O₂ in its standard state contributes zero. Therefore ΔH = 2(−396) − 2(−297) = −198 kJ for the reaction as written.

B: 85.48 kJ mol⁻¹

Use ΔG = ΔH − TΔS with consistent units. At 25 °C, T ≈ 298 K and ΔS = 0.0118 kJ mol⁻¹ K⁻¹. Therefore ΔG = 89 − 298(0.0118) = 85.4836 kJ mol⁻¹, or 85.48 kJ mol⁻¹.

C: n + m

The overall reaction order is the sum of the exponents of concentration in the experimentally determined rate law. For rate = k[X]ⁿ[Y]ᵐ, the order is n + m.

A: increase temperature at constant pressure

Increasing temperature favours the endothermic direction, so it increases the equilibrium proportion of products for an endothermic forward reaction. A pressure change depends on the numbers of gas particles on each side.

D: Disinfecting water

The reaction produces chlorine: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O. Chlorine is used in water disinfection because it destroys many disease-causing microorganisms.

C: [G]ᵖ[H]ᑫ / ([E]ᵐ[F]ⁿ)

The concentration equilibrium expression places products above reactants, with each concentration raised to its stoichiometric coefficient. Thus Kc = [G]ᵖ[H]ᑫ / ([E]ᵐ[F]ⁿ).

A: SO₄²⁻(aq) + Ba²⁺(aq) → BaSO₄(s), in dilute HNO₃

Sulfate ions form a white precipitate of barium sulfate with Ba²⁺: Ba²⁺ + SO₄²⁻ → BaSO₄(s). Acidifying with dilute nitric acid helps exclude interfering carbonate precipitates.

B: Reaction of an acid with a base

Rust contains iron oxides and hydroxides. These basic materials react with acid to form iron salts and water, so their removal by acid is based on an acid–base reaction.

A: Electrolysis using mercury as the cathode

In the historical mercury-cell process, sodium forms an amalgam at the mercury cathode. The amalgam then reacts with water in a separate decomposer to produce NaOH and hydrogen, while mercury is recycled. This identifies the mercury-cathode electrolysis process.

B: Alkanoate

An alcohol and a carboxylic acid undergo acid-catalysed esterification to form an ester, also called an alkanoate, and water. Concentrated sulfuric acid provides the acid catalyst.

A: CH₃–CHI₂

The first addition gives CH₂=CHI. Further addition of HI produces CH₃CHI₂, with both iodine atoms on the same carbon. The overall reaction is HC≡CH + 2HI → CH₃CHI₂.

B: The calcium salts of detergents are water-soluble

Common anionic detergents form calcium and magnesium salts that remain soluble. Soap instead forms insoluble salts with these ions, producing scum and reducing its cleaning action in hard water.

C: CH₃COOH

KOH provides 0.025 × 0.02 = 0.00050 mol OH⁻. A monobasic acid reacts with it in a 1:1 ratio, giving molar mass 0.030/0.00050 = 60 g mol⁻¹. CH₃COOH has this molar mass: 2(12) + 4(1) + 2(16) = 60.

B: Migration

Chromatographic components interact differently with the stationary and mobile phases. They therefore travel at different rates, or rates of migration, which separates the mixture.

B: Rusting of iron

Rusting produces new iron-containing oxide and hydroxide substances through reaction with oxygen and water. This is a chemical change. Melting and distillation change physical state or separate substances without this type of chemical transformation.

B: 6.02 × 10²²

M(H₂SO₄) = 2 + 32 + 4 × 16 = 98 g mol⁻¹. Thus 4.9 g is 0.050 mol acid. Complete release of both acidic hydrogens gives 0.100 mol H⁺, or 0.100 × 6.02 × 10²³ = 6.02 × 10²² ions.

D: 16 cm³

The reaction is 2H₂ + O₂ → 2H₂O. At the same temperature and pressure, 8 cm³ of hydrogen consumes 4 cm³ of oxygen. The oxygen left is 20 − 4 = 16 cm³.

A: T

T has atomic number 16, so it represents sulfur, a solid nonmetal at room temperature. R (13) is aluminium, J (8) is oxygen and X (4) is beryllium. The two metals and gaseous oxygen do not satisfy the description.

C: Charge of the electron

The oil-drop experiment measured electric charges on droplets and showed that they occurred in multiples of a fundamental charge. This determined the magnitude of the electron’s charge.

B: crystal lattice forces

Oppositely charged ions attract one another throughout an ionic crystal lattice. These electrostatic lattice forces account for much of the stability of ionic solids. Electron-pair sharing instead describes covalent bonding.

B: Light and noncombustible

Helium is less dense than air, providing buoyancy, and it is noncombustible. Its chemically unreactive nature makes it suitable as a lifting gas.

C: Hygroscopic

A hygroscopic substance absorbs water from the surrounding air. A deliquescent substance absorbs enough water to dissolve in it, so deliquescence is a special case of hygroscopic behaviour.

D: do not settle out on standing, whereas suspensions do

Colloidal particles are small enough to remain dispersed under ordinary standing conditions. The larger particles of a suspension settle under gravity. Colloids also scatter light, so lack of light scattering is not their defining distinction.

D: NH₄Cl(aq)

Ammonium ions are weak acids: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. This makes NH₄Cl solution acidic. Sodium chloride is approximately neutral, while sodium carbonate is basic.

C: 9.4

NaOH dissociates to give [OH⁻] ≈ 2.50 × 10⁻⁵ mol dm⁻³. Therefore pOH = −log₁₀(2.50 × 10⁻⁵) = 4.60. At 25 °C, pH = 14.00 − 4.60 = 9.40.

B: Weak acid versus strong base

The solution begins weakly acidic, and the sharp rise occurs above pH 7. This is the characteristic pattern for titrating a weak acid with a strong base: the conjugate base makes the equivalence mixture alkaline.

D: Cathode, using direct current

Silver ions gain electrons and deposit as metal at the cathode: Ag⁺ + e⁻ → Ag. The object to be plated must therefore be the cathode, and a direct current maintains the required electrode polarity.

C: 1.5

Copper(II) ions require two electrons per atom: Cu²⁺ + 2e⁻ → Cu. A charge of 3 faradays supplies 3 mol of electrons, so it deposits 3/2 = 1.5 mol of copper.

B: Oxidation

Chloride ions lose electrons in 2Cl⁻ → Cl₂ + 2e⁻. Electron loss is oxidation, and chlorine’s oxidation number increases from −1 to 0.

A: 0

Oxygen has oxidation number −2 both in Cr₂O₇²⁻ and in H₂O. It does not change oxidation state, so the change is zero. Chromium and iodine are the elements undergoing redox changes.

A: Low temperature

A negative ΔH means the forward reaction releases heat. Lowering temperature favours this exothermic direction, increasing the equilibrium proportion of products. This describes the equilibrium shift, not the speed of reaction.

B: For path II is higher than for path I

Path II has a lower activation-energy barrier than path I. For the catalysed pathway of the same reaction under comparable conditions, this allows more successful reaction events and a faster rate. A catalyst leaves the reactant and product energies unchanged.

A: Washing under pressure

Washing the compressed gas with water removes much of the CO₂ because its solubility increases with pressure. Ammoniacal copper(I) chloride is used for CO rather than CO₂; a drying agent removes water.

C: Zn²⁺

Zn²⁺ first gives white Zn(OH)₂ with hydroxide ions. In excess NaOH, the precipitate dissolves to form [Zn(OH)₄]²⁻. Copper(II) hydroxide is blue, while calcium and magnesium hydroxides do not show this amphoteric dissolution.

D: Remove impurities

Limestone decomposes to CaO. This basic oxide reacts with acidic impurities such as SiO₂ to form calcium silicate slag: CaO + SiO₂ → CaSiO₃. Removing these impurities is the purpose of the limestone flux.

C: CaCl₂

Calcium compounds produce an orange-red or brick-red flame. Sodium gives yellow, lithium crimson, and magnesium salts give no characteristic visible flame-test colour.

A: Propanal and propanone

Propanal and propanone both have molecular formula C₃H₆O. They differ in connectivity and functional group: propanal is an aldehyde and propanone a ketone. Each other pair has different molecular formulae.

B: Dibutylamine

Dibutylamine has two butyl groups bonded to nitrogen and one N–H bond, so it is a secondary amine. Methylamine is primary, trimethylamine is tertiary and propylene is a hydrocarbon.

D: Hydrogen bonding

Alcohol molecules form intermolecular hydrogen bonds through their O–H groups. More energy is needed to separate the molecules, giving relatively high boiling points compared with similar-sized molecules without this hydrogen bonding.

B: 11.2 dm³

CH₄ has molar mass 16 g mol⁻¹, so 8 g contains 0.5 mol. CH₃CH₂CH₃ has molar mass 44 g mol⁻¹, so 22 g also contains 0.5 mol. Equal amounts of gas occupy equal volumes at the same temperature and pressure: 11.2 dm³.

D: 1092 K

For a fixed amount of gas, PV/T is constant. Doubling both pressure and volume multiplies PV by four, so the absolute temperature must become 4 × 273 = 1092 K.

B: The vapour density of the gas

Vapour density relative to hydrogen compares the masses of equal gas volumes at the same temperature and pressure. Since hydrogen has relative molecular mass 2, Mr = 2 × vapour density. Thus Y is the vapour density.

C: 2.0

By Graham’s law, diffusion rate is inversely proportional to the square root of gas density under the same conditions. Hence rate(X)/rate(Y) = √(2.0/0.5) = √4 = 2.0.

B: The gas molecules bombard the container walls more frequently

Heating increases the average kinetic energy of the gas molecules. In a fixed-volume container they strike its walls more frequently and with greater momentum, increasing the pressure.

D: Electronegativity

Down a group, additional electron shells increase shielding and the distance between the nucleus and bonding electrons. Electronegativity therefore generally decreases. Atomic size and electropositivity generally increase.

C: 1, 1, 1

Deuterium is hydrogen-2. A neutral deuterium atom has one proton, one neutron and one electron, giving mass number 2 and atomic number 1.

B: Water vapour in air

Anhydrous calcium chloride absorbs water vapour. Passing a known volume of air through the tube allows the absorbed water to be determined from its mass gain, so the apparatus measures water vapour in air.

B: CaO

Calcium oxide is a basic oxide: it neutralises acids to form a salt and water, for example CaO + 2HCl → CaCl₂ + H₂O. CO₂ is an acidic oxide, while H₃PO₃ and CH₃COOH are acids.

D: 0.560 dm³

Producing one mole of O₂ from water or hydroxide requires four moles of electrons. The charge supplies 9650/96500 = 0.100 mol electrons, giving 0.0250 mol O₂. At s.t.p., its volume is 0.0250 × 22.4 = 0.560 dm³.

C: The anode

The impure copper is the anode, where copper dissolves by oxidation: Cu → Cu²⁺ + 2e⁻. Copper ions are reduced and deposited on a separate cathode. This transfers copper away from the impure electrode.

B: The hydride ion is a reducing agent

Hydrogen in H⁻ has oxidation number −1 and becomes 0 in H₂. The hydride ion is oxidised, so it acts as the reducing agent. Hydrogen from water is reduced from +1 to 0.

A: +7 to +2

In MnO₄⁻, four oxygen atoms contribute −8. The ion has charge −1, so manganese is +7. In Mn²⁺ it is +2, giving a change from +7 to +2.

B: Increasing temperature decreases the equilibrium constant

For an exothermic forward reaction, increasing temperature favours the reverse, endothermic direction. The equilibrium constant for the forward reaction decreases. A catalyst changes the rate of reaching equilibrium but not the equilibrium constant.

A: N₂O and steam

The controlled thermal decomposition represented at this level is NH₄NO₃ → N₂O + 2H₂O. Its gaseous products are nitrous oxide and steam.

B: Sodium sulfate

Sulfate ions form white barium sulfate, which remains insoluble in dilute hydrochloric acid: Ba²⁺ + SO₄²⁻ → BaSO₄(s). Acid removes interference from carbonate and sulfite by converting them into gaseous products.

A: The reaction of SO₃ with water is violently exothermic

The direct reaction of SO₃ with water releases intense heat and produces a fine sulfuric-acid mist that is difficult to collect. In the Contact process, SO₃ is instead absorbed in concentrated sulfuric acid before controlled dilution.

A: Made the cathode

Cathodic protection supplies electrons to the iron so that it does not undergo the anodic oxidation Fe → Fe²⁺ + 2e⁻. The protected iron is made the cathode.

D: Sodium ethoxide and hydrogen

Sodium replaces the hydroxyl hydrogen of ethanol to form sodium ethoxide and hydrogen gas: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. The oxygen remains part of the ethoxide ion.

B: R₂CO

An alkanone has a carbonyl carbon bonded to two alkyl groups, represented by R₂CO. The two alkyl groups may be the same or different. RCHO represents an aldehyde, RCOOH a carboxylic acid and RCOOR an ester.

A: CH₃COOH

Sulfuric acid protonates the ethanoate ion, releasing ethanoic acid: CH₃COONa + H₂SO₄ → CH₃COOH + NaHSO₄. This is an acid–base reaction.

C: 2s and 2p

One carbon 2s orbital and three carbon 2p orbitals form four sp³ hybrid orbitals in the bonding model for methane. Each overlaps with a hydrogen 1s orbital to form a C–H sigma bond.

B: Bronze

Bronze is an alloy containing copper and tin, often with smaller amounts of other elements. Its components are mixed rather than combined in one fixed molecular ratio, so it is a mixture. Pure sulfur, distilled water and ethanol are single substances.

C: 3.75 mol

The balanced equation is 2KClO₃ → 2KCl + 3O₂. Two moles of chlorate produce three moles of oxygen, so 2.5 mol produces 2.5×3/2 = 3.75 mol of O₂.

A: 1.88 dm³

At constant pressure, volume is proportional to absolute temperature. Convert 25 °C and 100 °C to about 298 K and 373 K. Then V₂ = 1.50×373/298 ≈ 1.88 dm³. Celsius temperatures cannot be used directly in the ratio.

A: EX₃ and EX₅

The configuration contains 15 electrons, identifying phosphorus. Phosphorus can form trihalides and pentahalides, for example PCl₃ and PCl₅; the possible formulae are therefore EX₃ and EX₅.

C: isotopes

Both uranium atoms have atomic number 92, so each has 92 protons. Their mass numbers differ: 235 − 92 = 143 neutrons and 238 − 92 = 146 neutrons. Atoms of the same element with different neutron numbers are isotopes.

B: increases

A greater electronegativity difference makes the bonding electrons more unevenly shared. The partial charges and bond polarity therefore increase.

A: hydrogen bonding

Hydrogen bonds attract neighbouring water molecules to one another. Boiling requires overcoming these intermolecular attractions, giving water a relatively high boiling point for its small molecular mass.

A: Anhydrous copper(II) tetraoxosulfate(VI)

Anhydrous copper(II) sulfate is white and becomes blue when hydrated. This colour change provides a test for water.

D: Efflorescence

Efflorescence is the loss of water of crystallisation from a hydrated salt on exposure to air. Sodium carbonate decahydrate is a familiar example.

B: KNO₃

The KNO₃ curve has the steepest positive slope among the listed salts. A larger slope means a larger increase in solubility for the same rise in temperature.

C: H3O+ acts as an acid

H₃O⁺ donates a proton to NH₃, forming NH₄⁺ and H₂O. A Brønsted–Lowry acid is a proton donor, so hydronium acts as the acid in this reaction.

A: 0.40 mol dm⁻³

The molar mass of NaOH is 23 + 16 + 1 = 40 g mol⁻¹. Thus 4.0 g is 0.10 mol. The solution volume is 250 cm³ = 0.250 dm³, giving 0.10/0.250 = 0.40 mol dm⁻³.

C: Ba

The more negative the standard reduction potential, the greater the tendency of the metal to be oxidised in the reverse process. Barium has the most negative value, −2.90 V, so it is the strongest reducing agent in this comparison.

D: +1, +5 and +7

Take hydrogen as +1 and oxygen as −2. Each neutral molecule has total oxidation number zero: chlorine is +1 in HOCl, +5 in HClO₃ and +7 in HClO₄.

A: −42 kJ mol⁻¹

Use ΔH° = sum of product formation enthalpies minus sum of reactant formation enthalpies. H₂(g) has zero standard formation enthalpy, so ΔH° = (−394 + 0) − (−110 − 242) = −42 kJ mol⁻¹.

A: Remain the same

At a fixed temperature, the equilibrium vapour pressure of a pure solid is independent of how much solid is present, provided some solid remains. Adding 1 g therefore leaves the equilibrium vapour pressure unchanged.

C: Addition of a catalyst

A catalyst provides a reaction pathway with a lower activation-energy barrier. It does not change the energies of the reactants and products, so the reaction enthalpy remains the same.

A: A strong acid

Sulfuric acid donates a proton to chloride, producing hydrogen chloride and hydrogensulfate. It is acting as an acid. No water is removed in this equation, and the oxidation states do not change.

A: Silver nitrate

Silver nitrate decomposes to silver, nitrogen dioxide and oxygen: 2AgNO₃ → 2Ag + 2NO₂ + O₂. Sodium nitrate gives a nitrite and oxygen, while the calcium and lithium salts give their metal oxides rather than the metals.

C: Bubble it through sodium hydroxide solution

Sodium hydroxide absorbs the acidic oxide CO₂, while CO does not react in this washing step. With sufficient hydroxide, CO₂ + 2NaOH → Na₂CO₃ + H₂O. Passing the mixture over hot CuO would instead consume CO by oxidising it to CO₂.

B: They react with aqueous AgNO₃ to give a white precipitate soluble in excess ammonia

Chloride ions give a white silver chloride precipitate with aqueous silver nitrate: Ag⁺ + Cl⁻ → AgCl(s). Excess ammonia dissolves it by forming the soluble complex [Ag(NH₃)₂]⁺.

D: PbBr₂(s), K⁺(aq) and NO₃⁻(aq)

Lead(II) ions and bromide ions form insoluble lead(II) bromide: Pb²⁺ + 2Br⁻ → PbBr₂(s). Potassium and nitrate ions remain in solution as spectator ions. The full reaction is Pb(NO₃)₂ + 2KBr → PbBr₂ + 2KNO₃.

E: Copper and tin

Traditional bronze is an alloy of copper and tin. Copper and zinc form brass, which is a different alloy.

D: Cu(NO₃)₂, NO₂ and H₂O

Concentrated nitric acid oxidises copper to copper(II) nitrate and is reduced mainly to nitrogen dioxide. The balanced equation is Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O.

B: an amphoteric oxide

Aluminium oxide reacts with sulfuric acid and also with sodium hydroxide. An oxide that reacts with both acids and bases is amphoteric.

C: kerosene

The kerosene fraction is used to produce common aviation turbine fuels such as Jet A and Jet A-1. Refinery gas and gasoline are lighter fractions, while diesel is a heavier fuel fraction.

D: 2,4-dimethylhexane

The longest carbon chain has six atoms. Numbering from the end nearer the first branch puts methyl groups at positions 2 and 4, rather than 3 and 5. The name is 2,4-dimethylhexane.

B: It causes pollution through exhaust emissions

Tetraethyllead improves resistance to engine knocking, but burning leaded fuel releases lead-containing pollution in the exhaust. This environmental contamination is the reason for avoiding it as an antiknock additive.

B: sp³ hybridised

Each ethane carbon forms four sigma bonds, three to hydrogen and one to the other carbon. This approximately tetrahedral arrangement corresponds to sp³ hybridisation.

D: Microbial activity produces organic acids in it

Microorganisms produce organic acids during continued palm-wine fermentation. For example, acetic-acid bacteria can oxidise ethanol to ethanoic acid, increasing acidity and producing a sour taste.

D: Alkanoate esters reacting with sodium hydroxide

Saponification is alkaline hydrolysis of an ester. An alkanoate ester reacts with sodium hydroxide to form a sodium carboxylate and an alcohol: RCOOR′ + NaOH → RCOONa + R′OH. Fats give soap and glycerol because they are glycerol esters.

C: Metallic cohesion remains in the liquid, while boiling separates the atoms

Liquid metal still has strong attraction between its atoms and delocalised electrons. Melting permits atoms to move past one another, while boiling separates them into the gas phase and overcomes much more of this cohesion. This helps explain why vaporisation needs much more energy.

C: 3, 2, 6, 1

One Al₂(SO₄)₃ unit requires two aluminium atoms and three sulfate groups, giving coefficients 2 for Al(OH)₃ and 3 for H₂SO₄. Their 12 hydrogen atoms form 6H₂O. The balanced coefficients are 3, 2, 6 and 1.

B: 12.5 s

For equal volumes under the same conditions, diffusion time is proportional to the square root of molar mass. Thus t(H₂) = 50×√(2/32) = 12.5 s. Hydrogen has one-sixteenth the molar mass of oxygen and diffuses four times as fast.

D: Temperature

Temperature indicates average molecular kinetic energy. In the ideal-gas model, average translational kinetic energy is proportional to absolute temperature. Volume, mass and pressure are not direct measures of this average energy.

C: Two neutrons and one proton

Tritium is hydrogen-3. Hydrogen has atomic number 1, so its nucleus contains one proton. Mass number 3 means there are two additional nucleons, both neutrons. The electron of a neutral tritium atom is outside its nucleus.

B: A helium nucleus

Conservation of mass number gives 14 + A = 17 + 1, so A = 4. Conservation of atomic number gives 7 + Z = 8 + 1, so Z = 2. X is therefore ⁴₂He, an alpha particle or helium nucleus.

D: S

Seven outer-shell electrons are characteristic of the halogens, which are predominantly nonmetals. They tend to gain one electron or share electrons to complete the shell. Having one, two or three outer-shell electrons is more commonly associated with metallic main-group elements.

D: Sulfur dioxide

Sulfur dioxide can dissolve in atmospheric water and undergo further oxidation to sulfuric acid. This contributes to acid rain. Oxygen, nitrogen and carbon monoxide are not the listed acid-rain precursor in this comparison.

B: Bacteria

Chlorination disinfects water by inactivating bacteria and other susceptible microorganisms. It does not remove the dissolved calcium and magnesium ions responsible for hardness. Effective treatment also depends on dose, contact time and water conditions.

A: I, II, III and IV

Gas solubility depends on how the gas and solvent interact, so both chemical identities matter. Temperature changes dissolution equilibrium, and the gas’s partial pressure also affects how much dissolves. All four listed factors can therefore affect solubility.

C: NaHS

NaHS is sodium hydrogen sulfide. It results from replacing only one of the two acidic hydrogens of H₂S with sodium, leaving a hydrogen in HS⁻. This makes it an acid salt by composition; that classification does not mean its aqueous solution must have pH below 7.

B: 103.0 g

The required amount is concentration×volume = 6.00×0.175 = 1.05 mol. H₂SO₄ has molar mass 2(1.00) + 32.06 + 4(16.00) = 98.06 g mol⁻¹. The mass is 1.05×98.06 = 102.963 g, approximately 103.0 g.

B: Oxygen and copper

At the cathode, Cu²⁺ gains electrons and deposits as copper. At the carbon anode, water is oxidised and oxygen is evolved. Thus the anode product is oxygen and the cathode product is copper.

B: 5.4 kg

The charge is 500×24×3600 = 43,200,000 C. Each Mg²⁺ ion needs two electrons, so the magnesium amount is Q/(2F) = 43,200,000/(2×96,500) ≈ 223.8 mol. Its mass is 223.8×24/1000 ≈ 5.4 kg.

C: −2, +1, 0

Manganese changes from +4 in MnO₂ to +2 in Mn²⁺, a change of −2. Chlorine changes from −1 in Cl⁻ to 0 in Cl₂, a change of +1 per atom. Hydrogen stays at +1, so its change is 0. Reduction means manganese gains electrons.

B: I₂

I₂ gains two electrons to form 2I⁻, so iodine is reduced. The species that is reduced is the oxidising agent. Thiosulfate donates electrons and is oxidised to tetrathionate.

B: Their product is 1

Reversing a reaction swaps the numerator and denominator of its equilibrium expression. At the same temperature, K_reverse = 1/K_forward. Multiplying the two constants therefore gives 1.

A: H₂(g) + I₂(g) ⇌ 2HI(g)

H₂(g) + I₂(g) ⇌ 2HI(g) has two moles of gas on each side. Compressing the system changes all concentrations by the same factor, which cancels in the reaction quotient. The other listed equilibria have unequal gas-mole totals and shift when volume changes at constant temperature.

C: ([R]ᵐ[S]ⁿ)/([P]ˣ[Q]ʸ)

For xP + yQ ⇌ mR + nS, the concentration expression places product concentrations in the numerator and reactant concentrations in the denominator. Each concentration is raised to its coefficient: Kc = [R]ᵐ[S]ⁿ/([P]ˣ[Q]ʸ).

C: I and III

Oxygen with ethyne forms the oxyacetylene flame used in gas welding. Oxygen with hydrogen forms an oxyhydrogen flame that can join suitable lower-melting metals. Hydrogen and ethyne alone are both fuels and do not supply the oxidising gas needed for these conventional torch combinations.

D: ammonia

Hydroxide ions remove a proton from ammonium ions, releasing ammonia: NH₄⁺ + OH⁻ → NH₃ + H₂O. The full equation is NH₄NO₃ + NaOH → NaNO₃ + NH₃ + H₂O.

B: carbonate

Carbonate ions form insoluble white barium carbonate: Ba²⁺ + CO₃²⁻ → BaCO₃(s). The listed nitrate, chloride and sulfide alternatives do not give this barium precipitate under the usual aqueous test conditions.

D: Sodium carbonate

Sodium carbonate is the soda-ash product of the Solvay process. Brine supplies sodium chloride and limestone supplies calcium carbonate; ammonia is used and regenerated within the process.

D: manganese and magnesium.

Duralumin is an aluminium alloy whose characteristic alloying elements include copper, magnesium and manganese. Thus manganese and magnesium complete the list.

B: Slaking

Slaking is the reaction of quicklime, CaO, with water to form slaked lime, Ca(OH)₂. The negative enthalpy change means heat is released.

A: sp³ hybridised

Each carbon atom in ethane makes four sigma bonds: three C–H bonds and one C–C bond. Its four bonding directions are approximately tetrahedral, described by sp³ hybridisation.

D: 3,6-dimethylhept-3-ene

The longest chain containing the double bond has seven carbon atoms. Numbering from the nearer end gives the double bond locant 3 and methyl substituents at carbons 3 and 6. The name is 3,6-dimethylhept-3-ene.

A: CH₃CH₂CH(OH)CH₃

In CH₃CH₂CH(OH)CH₃, the carbon bearing the hydroxyl group is bonded to two other carbon atoms, so it is secondary. (CH₃)₃COH is tertiary, CH₃CH₂CH₂CH₂OH is primary, and CH₃CH₂OCH₂CH₃ is an ether.

A: Ethanol and dimethyl ether

Ethanol and dimethyl ether both have molecular formula C₂H₆O, but different connectivity: CH₃CH₂OH and CH₃OCH₃. They are functional-group isomers. The other pairs have different molecular formulae.

D: Carboxyl group

A carboxylic acid reacts with sodium hydrogencarbonate to release carbon dioxide bubbles: RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂. This effervescence identifies the carboxyl group among the choices.

D: Tollens’ reagent

Tollens’ reagent is reduced to metallic silver by an alkanal, producing a silver mirror or silver deposit. Ordinary alkanones do not give this test. Both classes can react with the other carbonyl reagents listed.

D: Starch

Starch contains many glucose units joined by glycosidic bonds, making it a polysaccharide. Glucose (also called dextrose) and mannose are monosaccharides.

A: Nitrogen from liquid air

Liquid air contains substances with different boiling points. Fractional distillation separates its components, allowing nitrogen to be collected separately from oxygen and other gases. The other listed tasks chiefly involve separating dissolved solids from their solvents.

B: 39.2 kg

The ore contains 80×0.700 = 56.0 kg of Fe₂O₃. Iron contributes 2×56 = 112 of its relative formula mass 160. The theoretical iron mass is therefore 56.0×112/160 = 39.2 kg.

C: Constant composition

The chlorine-to-metal mass ratios are 0.36/0.20 = 1.80 and 0.71/0.40 = 1.775. These are approximately the same at the precision of the reported masses. The data therefore support constant composition: the elements combine in essentially the same mass proportion.

C: 112 dm³

The balanced decomposition is 2KHCO₃ → K₂CO₃ + H₂O + CO₂. KHCO₃ has molar mass 100 g mol⁻¹, so 1 kg is 10 mol. Every two moles give one mole of CO₂; 5 mol therefore occupy 5×22.4 = 112 dm³ at STP.

A: It has formula XY

In the expected electron-transfer model, X loses two electrons to form X²⁺ and each Y gains one to form Y⁻. Charge balance requires two Y⁻ ions for each X²⁺, giving XY₂. A formula of XY would not be electrically neutral.

D: 9 and 11

An X⁻ ion has gained one electron, so its 10 electrons correspond to 9 protons. A Y⁺ ion has lost one electron, so its 10 electrons correspond to 11 protons. Ion formation changes electron count, not the number of protons in the nucleus.

C: Ionic and dative covalent

Ammonium chloride is an ionic solid of NH₄⁺ and Cl⁻ ions. Within NH₄⁺, the N–H bonds are covalent; formation from NH₃ and H⁺ can be described as donation of a nitrogen lone pair, or dative covalent bonding. Once NH₄⁺ has formed, its four N–H bonds are equivalent.

B: Sodium, magnesium, aluminium, phosphorus, chlorine

All five elements lie in period 3. Electronegativity generally increases from left to right across this period, so the order is Na < Mg < Al < P < Cl. Chlorine attracts shared bonding electrons most strongly in this list.

B: Hg²⁺

Mercury ions are toxic contaminants and are a concern in drinking water at low concentrations. Calcium and magnesium commonly contribute to water hardness, while iron can affect colour and taste. Mercury is the toxic-metal contaminant identified by this comparison.

B: 22.40 cm³

The carbonate amount is 0.1×(10/1000) = 0.001 mol. Acid converts each CO₃²⁻ into one CO₂ molecule, so the CO₂ amount is also 0.001 mol. Its STP volume is 0.001×22.4 = 0.0224 dm³, or 22.4 cm³.

B: It is redox: SO₂ is the oxidant and H₂S is the reductant

Sulfur in H₂S changes from −2 to 0, so H₂S is oxidised and acts as the reductant. Sulfur in SO₂ changes from +4 to 0, so SO₂ is reduced and acts as the oxidant. Two sulfide sulfur atoms each lose two electrons, balancing the four electrons gained by the SO₂ sulfur atom.

C: Lowering the activation energy

MnO₂ acts as a catalyst. It provides a reaction pathway with a lower activation energy, so more reacting particles can overcome the energy barrier. A catalyst does not change the overall enthalpy change of the reaction.

A: Low temperature and high pressure

The negative enthalpy change means the forward reaction releases heat, so lowering temperature favours products. There are 2 moles of reactant gases and 1.5 moles of product gases per equation. Higher pressure therefore also favours products, which occupy the smaller gas volume.

C: Four

The rate ratio is (1.2×10⁻⁴)/(3.0×10⁻⁵) = 4. Thus, when the listed initial NO concentration doubles, the measured initial rate increases fourfold.

C: N₂O

Nitrous oxide, N₂O, can support combustion and relight a glowing splint. At the hot splint it can decompose to nitrogen and oxygen. The glowing-splint observation is therefore not exclusive to a sample of pure oxygen.

A: Na₂CO₃

Sodium carbonate is thermally stable enough to melt during ordinary heating. Calcium, magnesium and zinc carbonates instead undergo thermal decomposition to their oxides and carbon dioxide before they can simply be melted under ordinary laboratory conditions.

C: Zinc forms a complex soluble in excess ammonia

Aqueous ammonia first supplies hydroxide ions, forming white Zn(OH)₂. In excess ammonia, zinc forms the soluble complex ion [Zn(NH₃)₄]²⁺, so the precipitate dissolves. This ligand-complex formation explains the observation.

C: Ca(OH)₂

A small amount of CO₂ reacts with limewater, Ca(OH)₂(aq), to form insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The CaCO₃ is the white precipitate. With prolonged excess CO₂, that precipitate can dissolve again.

C: Mercury and silver

Silver and mercury nitrates can ultimately yield their metals on strong heating because the corresponding oxides are thermally unstable. Copper and magnesium nitrates instead leave stable metal oxides under the usual comparison conditions. The pair is therefore mercury and silver.

B: But-2-ene

Each carbon of the double bond in but-2-ene has one H and one CH₃ group. Restricted rotation allows distinct cis and trans arrangements. But-1-ene and 2-methylbut-2-ene have identical groups on one double-bond carbon, while CH₂BrCl has no double bond.

D: 80 g

Propyne has molar mass 3×12 + 4×1 = 40 g mol⁻¹, so 10 g is 0.25 mol. Complete addition across its triple bond uses 2 mol of Br₂ per mole of propyne. The bromine mass is 0.25×2×160 = 80 g.

A: Ethanol

Ethene first adds sulfuric acid to form ethyl hydrogen sulfate. Adding water and warming hydrolyses this intermediate to ethanol and regenerates sulfuric acid. The overall change adds H and OH across the ethene double bond.

C: Their salts with hard-water ions remain soluble

Soap forms insoluble calcium and magnesium salts, seen as scum in hard water. Common soapless detergents avoid this precipitation because their corresponding salts remain sufficiently soluble. They therefore retain cleaning action better in hard water.

B: 1-Chloro-2-methylpropane

The longest carbon chain has three atoms. Numbering from CH₂Cl places chlorine at carbon 1 and the methyl branch at carbon 2. The compound is 1-chloro-2-methylpropane.

A: ClCH₂COOH + HCl

In the first chlorination step, one hydrogen on the methyl group of ethanoic acid is replaced by chlorine. The products are chloroethanoic acid, ClCH₂COOH, and HCl. The carboxyl group remains intact.

C: Dissolving sodium chloride in water

When sodium chloride dissolves, its ions disperse through the water. Evaporating the water can recover the salt, so this is classified as a physical change. The other processes involve chemical reactions that produce new substances.

A: Sublimation, condensation and freezing

X goes directly from solid to gas, which is sublimation. Y goes from gas to liquid, which is condensation. Z goes from liquid to solid, which is freezing.

C: 32 g of O₂

Using the relative atomic masses Mg = 24 and O = 16, 24 g of Mg contains one mole of atoms. A mole of oxygen molecules has mass 32 g, so it contains the same number of particles. The other choices each contain half a mole of molecules.

D: CO, H₂S, NO₂, SO₂

The relative molecular masses are CO = 28, H₂S = 34, NO₂ = 46 and SO₂ = 64. In the stated approximation, rate is proportional to 1/√M, so lighter gases diffuse faster. The order is CO, H₂S, NO₂, SO₂.

D: S

S has 19 protons and 20 neutrons, so its mass number is 39. Atomic number 19 is odd and identifies potassium, which commonly loses one electron to form K⁺. The other atoms do not satisfy all three conditions.

B: Valence

Valence, or valency, describes an atom’s combining capacity, commonly expressed as the number of bonds it forms in simple compounds. Atomic number counts protons, oxidation number is a formal electron-accounting value, and electronegativity describes attraction for bonding electrons.

C: 4.70%

Pb(NO₃)₂ + H₂S → PbS + 2HNO₃ gives one mole of PbS per mole of H₂S. The PbS molar mass is 207 + 32 = 239 g mol⁻¹, so the H₂S amount is 5.02/239 mol. Its STP volume is about 0.470 dm³, giving (0.470/10.0)×100 = 4.70% by volume.

C: Mercury(II) chloride

Mercury used as a cathode can enter process wastes. Some is oxidised to mercury chloride compounds, including mercury(II) chloride. This makes mercury compounds a characteristic contamination risk of the mercury-cell chlor-alkali process.

B: Solid or liquid particles dispersed in gas

Smoke is an aerosol: small solid particles and often liquid droplets are suspended in a gas. Soot particles and condensed droplets are dispersed through air, which acts as the continuous phase.

C: 100 g mol⁻¹

The 25.00 cm³ aliquot contains 0.02000×0.1 = 0.00200 mol of acid because the acid is monobasic. The full 250 cm³ contains ten times as much, or 0.0200 mol. Its molar mass is 2.0/0.0200 = 100 g mol⁻¹.

A: 4.0×10⁻⁵ mol dm⁻³

For the usual dilute-solution calculation, [H⁺] = 10⁻ᵖᴴ. With pH 4.398, [H⁺] = 10⁻⁴·³⁹⁸ ≈ 4.0×10⁻⁵ mol dm⁻³. A change of one pH unit corresponds to a tenfold concentration change.

A: 0.20 mol

Deposited nickel is proportional to charge. Increasing from 0.1 to 0.4 faraday deposits four times the mass: 4×2.95 = 11.8 g. The amount is 11.8/58.7 ≈ 0.20 mol.

B: +6 to +3

For Cr₂O₇²⁻, let chromium’s oxidation number be x: 2x + 7(−2) = −2, so x = +6. The product is Cr³⁺, whose oxidation number is +3. Chromium is therefore reduced from +6 to +3.

C: −848 kJ

Use formation enthalpies of products minus those of reactants. Aluminium and iron in their elemental reference states contribute zero. Thus ΔH = [−1670 + 2(0)] − [−822 + 2(0)] = −848 kJ. There is one mole of Al₂O₃ in the balanced equation.

A: Zinc has a more positive oxidation potential than iron

Zinc is oxidised more readily than iron and acts as the sacrificial anode. Its oxidation potential is more positive than iron’s, while its reduction potential is more negative. Electrons supplied by zinc oxidation keep the iron cathodic and help protect it from corrosion.

D: 5 g of powder at 50 °C

Powder exposes more calcium carbonate surface to the acid than lumps do. A higher temperature also increases the rate by increasing the frequency of effective collisions. Powder at 50 °C combines both rate-increasing factors.

D: Cl₂

Chlorine is denser than air and can enter below it, pushing air upward out of an upright gas jar. This is upward displacement of air. Hydrogen and ammonia are lighter than air; nitrogen monoxide reacts with oxygen in air and is unsuitable for this collection method.

C: Expose each gas to air

Nitrogen monoxide, NO, reacts with oxygen in air to form brown NO₂: 2NO + O₂ → 2NO₂. This visible change distinguishes NO from the other three colourless gases in the list. Limewater alone cannot distinguish CO₂ from SO₂ because both can form white precipitates.

C: Iron

The conventional Haber process uses an iron catalyst to speed the reaction between nitrogen and hydrogen. A finely divided catalyst provides a large surface on which the reactant gases can adsorb and react.

C: Carbon monoxide can accumulate and cause poisoning

Incomplete combustion of coal can produce carbon monoxide. CO binds strongly to haemoglobin and reduces the blood’s ability to carry and release oxygen; it also interferes with cellular respiration. Dangerous CO can accumulate in a poorly ventilated room.

D: Calcium silicate

In the usual blast-furnace explanation, limestone supplies CaO, which reacts with silica impurity: CaO + SiO₂ → CaSiO₃. The calcium silicate joins the slag and is separated from the molten iron.

A: It readily absorbs water vapour from air

Solid sodium hydroxide absorbs moisture from air. It is strongly hygroscopic and can become wet as enough water is absorbed, changing the condition and concentration of the stored reagent. A tightly closed container limits this uptake.

D: Copper

Copper compounds can introduce colour into glass. Copper ions interact with light, producing characteristic colours that depend on composition and oxidation state. Potassium, barium and zinc do not provide the same characteristic transition-metal colouring in this comparison.

C: 2,4-Dimethylhex-3-ene

The longest chain containing the double bond has six carbons. The double bond is at position 3 from either end. Numbering from the nearer branched end gives methyl groups at positions 2 and 4, rather than 3 and 5, so the name is 2,4-dimethylhex-3-ene.

C: Alkanol

An alkanol has an O–H group that reacts with sodium to form an alkoxide and hydrogen gas. Alkanols are ordinarily neutral to litmus. Among the four listed classes, this combination of observations identifies an alkanol.

B: Neutralisation

Sodium hydroxide removes one acidic proton from oxalic acid. The products are sodium hydrogen oxalate and water: HOOC–COOH + NaOH → HOOC–COONa + H₂O. This acid–base reaction is neutralisation.

C: It melts over the range 300°C to 375°C

A broad melting interval supports the presence of more than one component. A pure crystalline substance normally melts sharply at a fixed pressure. Grinding, a density value and moisture absorption can also be properties of pure substances, so they do not by themselves establish a mixture.

D: C₆H₁₂

The empirical formula from C:H = 1:2 is CH₂, with mass 14. The vapour amount is 0.032/22.4 mol, giving molar mass 0.12/(0.032/22.4) = 84 g mol⁻¹. Since 84/14 = 6, the molecular formula is C₆H₁₂.

A: 737 mmHg

The collected gas contains hydrogen and water vapour. Dalton’s law gives total pressure = hydrogen pressure + water-vapour pressure. Therefore the hydrogen pressure is 760 − 23 = 737 mmHg.

C: The number of occupied electron shells increases from Li to Na to K

Lithium, sodium and potassium occupy successive periods in Group 1. Their occupied shells increase from two to three to four. The extra shells and shielding make the outer electrons farther from the nucleus, explaining the increase in atomic radius.

C: Y

For one mole of an ideal gas, PV = RT. Therefore PV/RT = 1 at every pressure, so the horizontal curve Y represents ideal-gas behaviour.

D: Y₂X₃

X has 8 electrons and is oxygen, which commonly forms X²⁻. Y has 13 electrons and is aluminium, which commonly forms Y³⁺. Two Y ions give +6 and three X ions give −6, so the neutral formula is Y₂X₃.

B: 55 protons and 78 neutrons

Atomic number counts protons, so the nucleus contains 55 protons. Mass number counts protons plus neutrons: neutrons = 133 − 55 = 78. Electrons occupy regions outside the nucleus.

B: Q

Atomic number 10 is neon. Its electron arrangement is 2,8, with a filled outer shell, so it is a noble gas. The other numbers identify beryllium, magnesium and silicon.

C: Smelting copper sulfide ores

Copper sulfide ores contain sulfur. During roasting and smelting, sulfur is oxidised and sulfur dioxide enters the process gases. Without effective capture, copper smelting is therefore a significant SO₂ emission source.

B: An amphoteric oxide

An amphoteric oxide reacts with both acids and bases. ZnO behaves as a base with acid, forming a zinc salt and water, and reacts with strong alkali to form soluble zincate species. This dual behaviour identifies it as amphoteric.

B: Differ by one proton

A Brønsted–Lowry acid forms its conjugate base by losing one proton, H⁺. For example, NH₄⁺ and NH₃ differ by one proton. Their charges differ by one unit, but they need not have opposite charges.

C: −4

Each cyanide ligand, CN⁻, contributes −1. With six cyanides and Z in oxidation state +2, the total charge is y = +2 + 6(−1) = −4. The anion is therefore [Z(CN)₆]⁴⁻.

D: +64 kJ mol⁻¹

Hess’s law gives ΔH(combustion) = 2ΔHf(CO₂) + 2ΔHf(H₂O) − ΔHf(C₂H₄), since O₂ has zero standard enthalpy of formation. Thus −1428 = 2(−396) + 2(−286) − x, giving x = +64 kJ mol⁻¹ with the supplied data.

D: IV only

There are two moles of gas on each side, so changing pressure does not favour either side in the ideal-gas model. Because the forward reaction is exothermic, higher temperature favours the reverse direction. Adding excess steam shifts equilibrium toward CO₂ and H₂, so only IV favours hydrogen formation.

D: Y

During melting, added heat changes solid into liquid while the temperature stays constant. Both phases coexist on the first horizontal section, labeled Y. The later horizontal section is the liquid-to-gas change.

B: Cu + 4HNO₃ → Cu(NO₃)₂ + 2H₂O + 2NO₂

Concentrated nitric acid oxidises copper to Cu²⁺ while nitrate is reduced mainly to NO₂. The balanced equation is Cu + 4HNO₃ → Cu(NO₃)₂ + 2H₂O + 2NO₂. Equation C describes the familiar dilute-acid reaction producing NO instead.

C: Vanadium(V) oxide

Vanadium(V) oxide, V₂O₅, catalyses the oxidation of SO₂ to SO₃ in the contact process. The SO₃ is then used to produce sulfuric acid. The catalyst increases the reaction rate without changing the equilibrium position.

D: Coke and ammoniacal liquor

Heating coal without air produces coke as the solid residue. Volatile products include ammonia, which dissolves in condensed water to form ammoniacal liquor. Coal tar and coal gas are other products of this process.

A: An oxidant

Potassium nitrate acts as the oxidising agent. It supports oxidation of the combustible components rather than acting as a solvent or as a catalyst. An oxidant is reduced while causing another substance to be oxidised.

C: Atmospheric carbon dioxide reacts with it and releases chlorine

Carbon dioxide in moist air reacts with bleaching powder, leading to loss of available chlorine. This lowers its bleaching strength during exposure. Atmospheric nitrogen does not displace chlorine in this way.

D: N₂O and water

Under controlled, gentle heating, ammonium nitrate decomposes to nitrous oxide and water: NH₄NO₃ → N₂O + 2H₂O. The nitrogen atoms total two on each side, and hydrogen and oxygen are also conserved.

A: 1,2-Dichloroethane

Chlorine adds across the carbon–carbon double bond of ethene. Each carbon receives one chlorine atom, forming CH₂Cl–CH₂Cl, or 1,2-dichloroethane. The carbon chain stays two atoms long.

C: Sulfur is chemically combined with the rubber

In sulfur vulcanisation, sulfur forms cross-links between rubber polymer chains. These links limit chain slippage and improve elastic recovery and mechanical strength. Joining isoprene monomers is polymerisation, which occurs before this treatment.

B: Neutralisation

Ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water: CH₃COOH + NaOH → CH₃COONa + H₂O. An acid reacting with a base in this way is neutralisation.

D: A hydrogen bond

The O–H hydrogen of one ethanoic acid molecule can be attracted to an oxygen lone pair on another molecule. This intermolecular attraction is a hydrogen bond. The covalent bonds are within each acid molecule.

D: Propane-1,2,3-triol

Fats and oils are esters of glycerol. Alkaline hydrolysis breaks their ester links to give fatty-acid salts, which are soaps, and glycerol. Glycerol has an OH group on each of its three carbons, so its name is propane-1,2,3-triol.

C: 3-Chloro-2-methylprop-1-ene

The longest chain containing the double bond has three carbons and is numbered to give that bond position 1. The methyl group is on carbon 2 and chlorine is on carbon 3. The name is therefore 3-chloro-2-methylprop-1-ene.

D: Methane

Methane can escape from coal seams and accumulate in mine air. A methane–air mixture can ignite and explode if an ignition source is present. Methane is therefore a major gas-explosion hazard in coal mines.

C: X

X burns with a sooty flame but does not readily decolourise bromine water, behaviour characteristic of an aromatic hydrocarbon such as benzene. Y decolourises bromine water, indicating a reactive multiple bond, while Z has the cleaner flame expected of a saturated hydrocarbon.

C: Sublimation

This laboratory process is conventionally described as sublimation. In detail, heated NH₄Cl dissociates reversibly into NH₃ and HCl gases, which recombine on the cooler surface to form solid ammonium chloride. Crystal formation is the return to the solid, rather than the solid-to-gas step.

C: M₄X₃

The combining capacities must balance. Four atoms of a trivalent element provide 4×3 = 12 valence units; three atoms of a tetravalent element provide 3×4 = 12. The simplest ratio is therefore M₄X₃.

B: 67.2 dm³

Complete combustion follows C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. At STP, water is condensed, so the gaseous product counted is 3 mol CO₂. Its volume is 3×22.4 = 67.2 dm³.

B: Charles’ law

The plot shows volume proportional to absolute temperature. For a fixed amount of gas at constant pressure, V/T is constant: Charles’ law. A pressure–temperature relationship at fixed volume describes a different gas law.

D: Van der Waals forces

Naphthalene is a molecular solid. Its non-polar molecules attract one another mainly through London dispersion forces, a type of van der Waals force. Melting overcomes these intermolecular attractions without breaking the covalent bonds within each molecule.

D: Atomic radius

Across a period, the number of protons increases while added electrons occupy the same main shell. The increasing effective nuclear attraction generally pulls electrons closer, decreasing atomic radius.

A: −1 and +7

Atomic number 17 identifies chlorine. It has oxidation state −1 in chlorides and +7 in perchlorates such as ClO₄⁻, where x + 4(−2) = −1 gives x = +7.

C: Electron affinity

Electron affinity describes the energy change when an electron is added to an isolated gaseous atom to form an anion. Ionisation energy instead concerns removing an electron; electronegativity concerns attraction for bonding electrons.

A: Na₂SO₄·7H₂O

Water lost is 1.34−0.71 = 0.63 g, or 0.63/18 = 0.035 mol. Anhydrous Na₂SO₄ has molar mass 142, so 0.71 g is 0.005 mol. The water-to-salt ratio is 0.035/0.005 = 7, giving Na₂SO₄·7H₂O.

A: Mg²⁺

Magnesium ions form insoluble salts with soap anions and prevent easy lather formation. Water that lathers readily therefore contains very little Mg²⁺ compared with hard water. Potassium and the listed carbonate species do not themselves cause this soap scum.

D: NaCl

NaCl is formed from a strong acid and a strong base. Its ions do not appreciably hydrolyse in water, so its solution is approximately neutral. NH₄Cl and FeCl₃ give acidic solutions, while Na₂CO₃ gives an alkaline solution.

D: 30.0 cm³

Complete neutralisation requires three moles of NaOH per mole H₃PO₄: H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O. NaOH amount is 0.0450×0.2 = 0.0090 mol, so acid amount is 0.0030 mol. Its volume is 0.0030/0.1 = 0.0300 dm³ = 30.0 cm³.

B: Mg(OH)Cl

A basic salt retains hydroxide groups from incomplete neutralisation of a base. Mg(OH)Cl contains both OH⁻ and Cl⁻, balancing Mg²⁺. This structural meaning differs from simply giving an alkaline solution, as Na₂CO₃ does.

A: Cu²⁺(aq) + 2e⁻ → Cu(s)

Reduction occurs at the cathode. Copper(II) ions accept two electrons each to form copper metal: Cu²⁺ + 2e⁻ → Cu. Chloride oxidation occurs at the anode, and copper dissolution is oxidation rather than cathodic reduction.

D: The enthalpy of solution is negative

Dissolving solid NaOH in water releases heat to the surroundings. The process is exothermic, so its enthalpy of solution is negative. A positive enthalpy would instead mean heat absorption.

B: Activation energy

A catalyst provides a reaction pathway with lower activation energy. It speeds both directions of a reversible reaction without changing the enthalpy change, free-energy change or equilibrium position.

B: Carbon dioxide

Carbon dioxide is used in CO₂ fire extinguishers. It does not burn and can reduce the oxygen concentration around a suitable fire. The other listed gases are not the extinguishing agent in this type of extinguisher.

C: It is only sparingly soluble in water

Carbon monoxide dissolves only slightly in water, so it can displace the water and collect in the inverted jar. Density relative to air is not the controlling property for collection over water.

C: Nitrogen(IV) oxide (NO₂)

Copper(II) nitrate releases nitrogen dioxide on heating: 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂. The equation conserves two copper atoms, four nitrogen atoms and twelve oxygen atoms. In NO₂, nitrogen has oxidation number +4, so the gas is nitrogen(IV) oxide.

B: Electrolysis of brine

In the chlor-alkali process, concentrated sodium chloride solution (brine) is electrolysed. Chloride ions are oxidised at the anode: 2Cl⁻ → Cl₂ + 2e⁻. This produces chlorine gas.

B: By electrolysing aluminium oxide dissolved in molten cryolite

Aluminium is too reactive for its oxide to be reduced commercially with coke. In the Hall–Héroult process, alumina is dissolved in molten cryolite and electrolysed. Aluminium ions gain electrons at the cathode to form aluminium metal.

B: The crude copper is made the anode

In electrolytic refining, impure copper is the anode and a sheet of pure copper is the cathode. Copper atoms at the anode lose electrons and enter solution as Cu²⁺. Copper ions gain electrons at the cathode, depositing purer metal. Some impurities remain dissolved and others form anode sludge.

B: Alkanols

An alkanoic acid reacts with an alkanol in esterification to form an alkyl alkanoate ester and water. Reaction with sodium instead forms an ionic carboxylate salt, not an alkyl ester.

B: Alkanes, alkenes, alkynes and aromatic hydrocarbons

Alkanes, alkenes, alkynes and aromatic hydrocarbons are names of classes. Ethane, ethene, ethyne and benzene are individual members, while methane through butane are all members of the alkane class.

D: A catalyst

Diastase is an enzyme preparation that speeds the hydrolysis of starch into smaller sugars such as maltose. Enzymes act as catalysts: they provide a faster reaction pathway and are regenerated during the catalytic process.

A: CH₃CH₂CH₂CH₂OH

Butan-1-ol has an O–H group, so its molecules form hydrogen bonds with one another. The aldehyde and ether cannot donate hydrogen bonds to their own molecules, and butane is non-polar. For these similarly sized molecules, this makes butan-1-ol the highest-boiling compound.

B: Destructive distillation

Destructive distillation heats coal with air excluded. The coal decomposes into coke and volatile products. Cooling and separating the volatile products yields coal tar and ammoniacal liquor, while coal gas remains gaseous.

B: Seawater

Seawater contains water together with dissolved salts and other substances. Its components are not combined in one fixed chemical ratio, so it is a mixture. Sugar and sodium chloride are compounds; iron is an element.

C: 0.413 atm

For an ideal gas at constant volume and temperature, P = nRT/V, so pressure is proportional to the amount of gas. Halving the amount halves the pressure: 0.825/2 = 0.4125 atm, which rounds to 0.413 atm.

D: Ethanal

At equal temperature and pressure, vapour density is proportional to molar mass. Ethanal, C₂H₄O, has molar mass 44; ethanoic acid and propanol each have 60, and dichloromethane has 85. Ethanal therefore has the lowest vapour density.

A: Ionic

Atomic numbers 12 and 9 identify magnesium and fluorine. Magnesium loses two electrons to form Mg²⁺, while two fluorine atoms each gain one electron to form F⁻. Their electrostatic attraction forms ionic MgF₂.

B: 16.2

Relative atomic mass is the abundance-weighted mean of isotopic masses: (0.90 × 16) + (0.10 × 18) = 16.2.

B: The bond polarity increases

A larger electronegativity difference causes the bonding electrons to be shared less equally. The resulting partial charges are larger, so the bond is more polar.

D: To coagulate fine mud particles

Alum helps fine suspended particles clump together into larger flocs that can settle or be filtered out. This coagulation step removes suspended matter; it is distinct from disinfection.

D: 13.7%

Na₃AsO₄ has molar mass 3×23 + 75 + 4×16 = 208 g mol⁻¹. The hydrate has mass 208 + 12×18 = 424 g mol⁻¹. Thus 38.9 g hydrate contains 38.9×208/424 = 19.08 g Na₃AsO₄. The solution mass is 100 + 38.9 = 138.9 g, giving 19.08/138.9×100 = 13.7%.

A: Adding NaHCO₃: gas evolves; no gas evolves

Lime juice contains acids that react with bicarbonate to release CO₂. Ethanol is not acidic enough to release CO₂ from sodium bicarbonate. Thus gas forms with lime juice but not with ethanol.

A: 0.094 mol dm⁻³

HCl and NaOH react in a 1:1 mole ratio. At equivalence, c(HCl) = 0.15 × 24.83/39.45 = 0.09441 mol dm⁻³, corresponding to 0.094 mol dm⁻³.

C: 2.24 g

The time is 2.5×3600 = 9000 s, so charge is 2×9000 = 18000 C. Mg²⁺ requires two electrons, giving mass = 18000×24/(2×96500) = 2.238 g, or 2.24 g.

C: 1.2 × 10²⁴

Copper in CuO has oxidation state +2 and gains two electrons to become Cu. One mole of copper therefore requires two moles of electrons: 2×6.0×10²³ = 1.2×10²⁴ electrons.

B: +25.6 kJ mol⁻¹

The molar mass of NH₄NO₃ is 80 g mol⁻¹, so 5 g is 0.0625 mol. Cooling the surroundings means the dissolution absorbs heat. Its enthalpy is +1.6/0.0625 = +25.6 kJ mol⁻¹.

B: −130 kJ mol⁻¹

By Hess’s law, reaction enthalpy equals product formation enthalpy minus the total for reactants: −811 − [(−395) + (−286)] = −130 kJ mol⁻¹. The negative sign means heat is released.

C: Decreasing the reaction temperature

The forward reaction is exothermic. Lowering the equilibrium temperature favours the heat-producing direction, increasing the equilibrium yield of SO₃. A catalyst speeds approach to equilibrium without changing the equilibrium yield.

B: 3Cl₂(g) + 6OH⁻(aq) → ClO₃⁻(aq) + 5Cl⁻(aq) + 3H₂O(l)

Hot concentrated alkali converts chlorine into chloride and chlorate(V): 3Cl₂ + 6OH⁻ → ClO₃⁻ + 5Cl⁻ + 3H₂O. Chlorine is both oxidised to +5 and reduced to −1, so the reaction is disproportionation.

A: Nitrogen

Magnesium reacts with nitrogen to form Mg₃N₂. Water hydrolyses this nitride: Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃. The ammonia gives dense white NH₄Cl fumes with HCl, identifying the original gas as nitrogen.

A: Plenty of cool running water

Immediately flush the affected skin with plenty of running water and remove contaminated clothing while rinsing. Alert the teacher and obtain medical attention. Do not try to neutralise the acid on the skin with an alkali; the reaction can add heat and worsen the injury.

D: Oxygen and sulfur

Oxygen has the allotropes O₂ and O₃. Sulfur also has distinct structural forms, including rhombic and monoclinic sulfur. Thus both elements in the oxygen–sulfur pair exhibit allotropy under ordinary conditions.

C: (ii) and (iv)

Ammonia and hydrogen chloride are highly soluble in water. When either dissolves rapidly inside the flask, the pressure falls and outside air pressure pushes water in, producing a fountain. Nitrogen and nitrous oxide are much less suitable.

C: Passing it directly through CaO

Heating an ammonium salt with calcium hydroxide releases ammonia. It can be dried by passing it directly through calcium oxide. Water absorbs ammonia, while concentrated sulfuric acid and calcium chloride react with it.

C: Iron, carbon and chromium

Stainless steel is an iron-based alloy containing carbon and chromium. Chromium forms a protective oxide layer that improves corrosion resistance. Some grades contain additional elements such as nickel.

D: Cooling a molten mixture of the required elements

A common way to make an alloy is to mix the constituent elements in a molten metallic mixture and then allow it to solidify. Mixing them before solidification distributes the components through the alloy.

D: All metals

All metals can suffer corrosion in a suitable environment, although their resistance differs greatly. Even noble metals can be attacked under particular conditions. Rusting is the specific corrosion of iron, not the name for all metal corrosion.

C: The 2s and 2p orbitals hybridise

Carbon has four valence electrons. In the valence-bond description of methane, one 2s orbital and three 2p orbitals mix to form four sp³ hybrid orbitals, each used in a C–H bond. This accounts for four equivalent tetrahedral bonds.

A: Branched-chain alkanes

Branched alkanes generally resist premature autoignition better than corresponding straight-chain alkanes. This increases knock resistance, allowing a petrol engine to operate more effectively at high compression.

B: Triiodomethane

Ethanol gives the iodoform reaction with alkaline iodine/hypoiodite. The yellow precipitate is CHI₃, called triiodomethane or iodoform. The other listed compounds are not the yellow solid formed in this test.

D: Ethane, methane and propane

Methane, ethane and propane are small hydrocarbons with very low boiling points. They belong to the light gaseous fraction. Petrol, kerosene and benzene boil at higher temperatures and are not all part of the most volatile fraction.

C: Alkali

The aqueous ash extract supplies alkaline substances. These react with fats in palm oil during saponification to form fatty-acid salts, which are soap. The palm oil supplies the fat, while the ash supplies the alkali.

A: 2-Methylbuta-1,3-diene

2-Methylbuta-1,3-diene is isoprene, a conjugated diene. Its molecules can polymerise into polyisoprene, a rubber. The conjugated double bonds make it a suitable rubber monomer.

D: Propanoic acid

Oxidation of a primary alcohol first gives an aldehyde and then a carboxylic acid when oxidation continues. Propan-1-ol therefore gives propanoic acid, retaining its three-carbon chain.

C: Evaporation

Evaporation occurs when molecules escape from a liquid surface into the gas phase. Condensation is the reverse change, from gas to liquid.

B: 2HF + N₂F₂ → 2NHF₂

At the same temperature and pressure, gas volumes are proportional to mole amounts. The ratio 10:5:10 simplifies to 2:1:2. Equation B has those coefficients and conserves two H atoms, two N atoms and four F atoms on each side.

A: 6.02 × 10²²

The molar mass of NaCl is 23 + 35.5 = 58.5 g mol⁻¹. Therefore 5.85 g contains 0.100 mol NaCl. Each formula unit contains one chlorine atom, so the number of chlorine atoms is 0.100 × 6.02 × 10²³ = 6.02 × 10²².

B: 1.5 g

Mg + 2HCl → MgCl₂ + H₂. The acid amount is 0.250 × 0.5 = 0.125 mol. Half as many moles of magnesium are needed: 0.0625 mol. Its mass is 0.0625 × 24 = 1.5 g.

B: Electrovalent, with valency 2

Atomic number 12 identifies magnesium. Its electron arrangement is 2,8,2, and it commonly loses two outer electrons to form Mg²⁺. It therefore forms ionic (electrovalent) compounds with valency 2.

D: 2.0 mol dm⁻³

The dissolved sugar amount is 34.2/342.0 = 0.100 mol. The original solution volume is 50/1000 = 0.050 dm³. Its saturated concentration is therefore 0.100/0.050 = 2.0 mol dm⁻³.

A: NaHSO₄

An acid salt retains replaceable hydrogen from partial neutralisation of a polyprotic acid. NaHSO₄ retains one acidic hydrogen from H₂SO₄. The other listed salts do not retain such a hydrogen. Acidity of a salt solution alone is a different concept.

D: 0.01 mol dm⁻³ sugar

Sugar dissolves mainly as neutral molecules and contributes essentially no mobile ions. Sodium hydroxide is a strong electrolyte, while hexanoic acid supplies some ions through partial ionisation. The sugar solution therefore conducts the least electricity.

D: 2.34 × 10⁴ C

Charge equals current multiplied by time: Q = It. One hour is 3600 seconds, so Q = 6.5 × 3600 = 23400 C = 2.34 × 10⁴ C.

D: Zn + 2HCl → ZnCl₂ + H₂

In Zn + 2HCl → ZnCl₂ + H₂, zinc changes oxidation state from 0 to +2 and hydrogen from +1 to 0. Electrons are transferred, making this a redox reaction. The other reactions do not change the elements’ oxidation states.

A: −51.0 kJ mol⁻¹

The base amount is 0.020 × 0.1 = 0.0020 mol. Since 102 J is released, the reaction enthalpy is negative: ΔH = −102/0.0020 = −51000 J mol⁻¹ = −51.0 kJ mol⁻¹.

C: 50 cm³

Air is approximately one-fifth oxygen by volume. The air volume is therefore about 10 ÷ 0.20 = 50 cm³. This is an approximate composition calculation.

D: Steam

Heated magnesium reacts with steam to produce magnesium oxide and hydrogen: Mg + H₂O(g) → MgO + H₂. Reaction with liquid water instead leads to magnesium hydroxide, so steam matches the stated oxide product.

C: Hydrogen and carbon monoxide

Hot carbon reduces steam: C + H₂O(g) → CO + H₂. The resulting mixture of carbon monoxide and hydrogen is called water gas.

A: They deplete oxygen needed by aquatic organisms

Microorganisms use dissolved oxygen while breaking down oxygen-demanding organic waste. If oxygen is consumed faster than it is replaced, less remains for fish and other aquatic organisms.

C: SO₂ and NO

Both gases can be oxidised further: 2SO₂ + O₂ → 2SO₃ and 2NO + O₂ → 2NO₂. Sulfur rises from oxidation state +4 to +6 and nitrogen from +2 to +4. The other pairs contain water or carbon dioxide, which do not undergo this ordinary further oxidation.

A: H and S; Cl

Hydrogen sulfide contains hydrogen and sulfur and forms black lead sulfide with lead(II) ethanoate. Chlorine contains only chlorine and bleaches moist litmus. These observations match the first pair.

C: CuS

Copper(II) sulfide is extremely insoluble and remains precipitated in dilute non-oxidising acid. Sodium sulfide, zinc sulfide and iron(II) sulfide react with dilute HCl to release hydrogen sulfide. Thus CuS is the acid-insoluble sulfide in the list.

C: O₂

Chlorine in water forms hydrochloric acid and hypochlorous acid. Sunlight promotes decomposition of hypochlorous acid, with overall reaction 2HOCl → 2HCl + O₂. The evolved gas is oxygen.

B: Al

Aluminium ions strongly hydrolyse carbonate in water, producing aluminium hydroxide and carbon dioxide rather than a stable simple aluminium carbonate. Iron(II), zinc and lead(II) form ordinary solid carbonates.

B: Quicklime

Quicklime, CaO, removes water by forming Ca(OH)₂ without consuming the ammonia. Ammonia reacts with common alternative drying agents such as concentrated sulfuric acid and calcium chloride, so quicklime is used.

B: 3, 8, 2

The four water molecules require eight hydrogen atoms, so y = 8. Nitrogen balance then gives 8 = 2x + z. Oxygen balance gives 24 = 6x + 4 + z. Subtracting these equations gives x = 3, then z = 2. The balanced equation is 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 4H₂O + 2NO.

B: Treatment with sodium hydroxide followed by filtration

Hot sodium hydroxide dissolves the aluminium-containing component as aluminate. The iron(III) oxide impurity remains insoluble and can be filtered off. This separates the impurity without dissolving it along with the aluminium.

C: K₂SO₄

Potassium ions give a lilac flame. Sulfate ions form a white barium sulfate precipitate with acidified barium chloride. K₂SO₄ provides both ions, while each other option fails at least one test.

D: Electrolysis of a molten salt

A very reactive metal is obtained by reducing its ions during electrolysis of a molten salt. In an aqueous salt solution, water can be reduced instead, and any highly reactive metal formed would react with the water.

B: Tetraethyllead

Tetraethyllead was historically added to petrol to suppress engine knocking by altering combustion chemistry. It is the antiknock additive among these choices.

B: Saponification

Saponification is the alkaline hydrolysis of fats and oils. Palm oil reacts with the alkaline solution to produce glycerol and salts of fatty acids; the fatty-acid salts are soap.

B: C₂H₅Cl

Stepwise replacement of methane hydrogen atoms by chlorine gives CH₃Cl, CH₂Cl₂, CHCl₃ and eventually CCl₄. These direct substitution products retain one carbon atom. C₂H₅Cl has two carbon atoms and is not one of them.

B: CₙH₂ₙ₊₁X

An acyclic alkane has formula CₙH₂ₙ₊₂. Replacing one hydrogen atom with a halogen atom X gives CₙH₂ₙ₊₁X, the formula for a saturated acyclic monohaloalkane.

B: Nylon and rubber

Nylon and rubber are polymers made by linking many small molecular units. Nylon can form by condensation polymerisation, while common synthetic rubbers form from unsaturated monomers. Soap and margarine are produced by different reactions.

B: Fermentation

Starch is first hydrolysed to fermentable sugars. Yeast then converts glucose into ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Distillation can subsequently separate the ethanol but does not create it.

B: Graphite

In graphite, each carbon atom bonds to three others and contributes an electron to a delocalised system. These mobile electrons carry electrical charge through the layers.

D: C₆H₁₂O

Take a 100 g sample. The amounts are 72/12 = 6 mol of carbon atoms, 12/1 = 12 mol of hydrogen atoms and 16/16 = 1 mol of oxygen atoms. The simplest whole-number ratio is 6:12:1, giving C₆H₁₂O.

E: four times as fast as oxygen

Graham’s law gives rate ∝ 1/√molar mass. For H₂ and O₂, rate(H₂)/rate(O₂) = √(32/2) = 4. Hydrogen therefore passes through the porous plug four times as fast under the same conditions.

C: 6.25 moles

Iron(III) oxide is Fe₂O₃, with formula mass 2×56 + 3×16 = 160. One kilogram is 1000 g, so the amount is 1000/160 = 6.25 mol.

C: (i) and (iv)

Sour taste and pH below 7 describe acidic behaviour. A typical alkali has pH above 7, can release ammonia from ammonium salts, and turns phenolphthalein pink. Thus (i) and (iv) are not typical alkali properties.

C: Ethanol

For these liquids, ethanol has the highest vapour pressure at 323 K. Its lower normal boiling point indicates greater volatility in this comparison. Vapour-pressure data confirm that its pressure at 323 K exceeds those of water, toluene and butan-2-ol.

B: slightly denser than sample 2

Removing oxygen and carbon dioxide from air leaves nitrogen mixed with noble gases, especially argon. Argon is heavier than nitrogen molecules, so this sample is slightly denser than chemically prepared pure nitrogen at the same temperature and pressure.

A: 0.457g

Charge passed is Q = It = 0.193 × (2 × 3600) = 1389.6 C. Cu²⁺ needs two electrons per copper atom, so moles of Cu = 1389.6/(2 × 96500) = 0.0072 mol. The deposited mass is 0.0072 × 63.5 = 0.4572 g, which is 0.457 g to three significant figures.

D: NaGa(OH)₄

A neutral formula must balance charges. Na contributes +1 and Ga contributes +3. Four hydroxide groups contribute −4, so Na[Ga(OH)₄] is neutral. The other listed formulas do not balance with Ga in oxidation state +3.

D: MnO₄⁻

MnO₄⁻ is reduced: manganese changes from +7 to +2, gaining electrons. The reactant that accepts electrons oxidises the other species and is the oxidising agent. Chloride instead loses electrons to form chlorine.

D: Zinc

Zinc is above iron in the reactivity series and more readily loses electrons. It displaces iron from iron(II) sulfate: Zn+Fe²⁺→Zn²⁺+Fe. Copper, mercury, silver and gold are below iron.

E: Ethane

Complete hydrogenation adds two H₂ molecules across the carbon–carbon triple bond: C₂H₂+2H₂→C₂H₆. The carbon skeleton still has two atoms, so the product is ethane.

B: Chlorine

Chlorine reacts with calcium hydroxide (slaked lime) to produce bleaching powder containing hypochlorite. The hypochlorite provides its oxidising and bleaching action.

E: the bombardment of the pollen grains by molecules of water.

Water molecules move randomly and continually collide with the pollen grains. The uneven impacts from different directions produce the grains’ irregular Brownian motion.

A: NaNO₂ and NaNO₃

Nitrogen dioxide disproportionates in alkali: 2NO₂+2NaOH→NaNO₂+NaNO₃+H₂O. Nitrogen starts at +4 and becomes +3 in nitrite and +5 in nitrate, so both salts form.

E: Barium carbonate

Ba²⁺ combines with CO₃²⁻ to form insoluble white BaCO₃. Sodium ions remain in solution: Ba(OH)₂+Na₂CO₃→BaCO₃+2NaOH.

D: −C(=O)OH

The carboxyl group is −C(=O)OH: a carbon atom bonded to both a carbonyl oxygen and a hydroxyl group. This combination distinguishes carboxylic acids from alcohols and ketones.

E: Hydrogen

Hydrogen is the most abundant chemical element in the universe. Much of its ordinary matter consists of hydrogen and helium, with heavier elements present in smaller proportions.

B: An alkane

An alkane burns completely to give CO₂ and water. It does not rapidly decolourise bromine without light-induced substitution. Alkenes and alkynes decolourise bromine by addition; the chlorinated alternatives would also produce chlorine-containing products.

D: CO₂ and H₂O

Carbon dioxide turns limewater milky by forming calcium carbonate. Water turns white anhydrous CuSO₄ blue as it becomes hydrated. Therefore Y is CO₂ and Z is H₂O.

E: Ca(OH)₂

Marble is mainly CaCO₃. Strong heating decomposes it to CaO and CO₂. The CaO reacts with water to form calcium hydroxide: CaO+H₂O→Ca(OH)₂, giving an alkaline solution.

C: Na₂S₂O₃·5H₂O

Acid decomposes thiosulfate into sulfur and sulfur dioxide: S₂O₃²⁻ + 2H⁺ → S + SO₂ + H₂O. Sulfur forms the yellow precipitate. Sulfur dioxide is a reducing agent and turns acidified dichromate from orange to green. Thus the salt is sodium thiosulfate pentahydrate.

A: hydrogenation

Hydrogenation adds hydrogen across carbon–carbon double bonds in unsaturated oil molecules. Increasing saturation can turn the liquid oil into a firmer fat suitable for margarine.

E: Pb²⁺

Pb²⁺ forms white Pb(OH)₂ with hydroxide ions. This amphoteric hydroxide dissolves in excess NaOH but remains in excess aqueous ammonia. A separate portion gives white PbCl₂ with dilute HCl. Aluminium chloride is soluble, so the chloride test distinguishes lead from aluminium.

D: It separates the soap from the glycerol-containing solution

Sodium chloride reduces the solubility of the soap in the aqueous mixture, causing the soap to separate. Glycerol remains in the liquid. This separation is called salting out.

C: To form links that bind rubber chains together

Sulfur forms cross-links between rubber polymer chains. These links limit how freely the chains slide past one another, making the rubber stronger and helping it recover its shape after stretching.

D: evaporation

Evaporation removes water from brine. The dissolved sodium chloride remains and crystallises as enough water is lost.

D: 1.12 dm³

NH₄NO₂ has formula mass 2×14 + 4×1 + 2×16 = 64. Its amount is 3.20/64 = 0.050 mol. The equation produces one mole of N₂ per mole of NH₄NO₂, so the nitrogen volume is 0.050×22.4 = 1.12 dm³.

B: 2 and 3

Oxygen and copper do not react with water or steam in this comparison. Hot carbon and zinc react with steam. Bromine reacts reversibly with water to form HBr and HOBr, so copper and bromine cannot be the correct pair.

E: become mobile as intermolecular forces holding the solid are overcome

On melting, naphthalene molecules gain enough energy to move out of their fixed crystal positions. The molecules remain chemically intact; it is the intermolecular attractions holding the solid structure that are overcome.

A: 2:1

Hydrogen is H₂: 2 g represents 2/2 = 1 mol of molecules. Oxygen is O₂: 16 g represents 16/32 = 0.5 mol. Molecule counts are proportional to moles, giving 1:0.5 = 2:1.

D: PbO₂ + 2HNO₃ + H₂O₂ → Pb(NO₃)₂ + 2H₂O + O₂

A reducing agent is itself oxidised. In D, oxygen in H₂O₂ changes from −1 to 0 in O₂, while lead changes from +4 in PbO₂ to +2 in Pb(NO₃)₂. In the other reactions peroxide is reduced to water.

C: I⁻ is oxidised to I₂

Iodide loses electrons: 2I⁻ → I₂ + 2e⁻. Its oxidation number rises from −1 to 0, so it is oxidised. Each Fe³⁺ ion gains an electron to become Fe²⁺, which is reduction.

D: Exothermic

The products lie below the reactants in energy, so the forward reaction releases energy. The approximate change is 40 − 60 = −20 kJ, indicating an exothermic reaction. The profile alone does not establish spontaneity.

D: −114.6 kJ

The reference reaction forms one mole of water and releases 57.3 kJ. The second equation forms two moles of water. In the stated dilute neutralisation approximation, ΔH = 2 × (−57.3) = −114.6 kJ. The negative sign indicates heat released.

A: Ca(OH)₂

Limewater is a clear aqueous solution of calcium hydroxide, Ca(OH)₂. Carbon dioxide produces suspended calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, making it milky.

B: CO is readily oxidised by air at room temperature to form CO₂

CO does not readily combine with air at room temperature without suitable activation or catalysis. It can burn to CO₂ when ignited. It is poisonous and can act as a reducing agent, so B is the false statement.

D: amphoteric

Zinc oxide reacts with a basic oxide, Na₂O, and with an acidic oxide, CO₂. An oxide that can show both acidic and basic behaviour is amphoteric.

D: CO

Carbon monoxide is classified as a neutral oxide in ordinary acid-base chemistry. CO₂, SO₂ and NO₂ are acidic oxides, while Al₂O₃ is amphoteric.

A: a reducing agent

Nitrogen changes from −3 in NH₃ to 0 in N₂, so ammonia is oxidised. The substance that is oxidised reduces the other reactant and is therefore the reducing agent.

C: a catalyst

Iron provides a catalytic surface that speeds up the reaction of nitrogen with hydrogen to make ammonia. It changes the rate of reaching equilibrium, not the equilibrium composition.

D: C₅H₇O₂N

Vapour density relative to hydrogen gives molecular mass 2×56.5 = 113. For 100 g, the approximate mole amounts are C:53.1/12, H:6.2/1, O:28.3/16 and N:12.4/14. Dividing by the smallest gives C:H:O:N ≈ 5:7:2:1. C₅H₇O₂N has formula mass 113, so it is also the molecular formula.

D: cracking

Cracking uses heat, often with a catalyst, to break larger hydrocarbon molecules into smaller ones. The smaller products generally have lower boiling points.

E: propanoic acid

CH₃CH₂C(=O)OH contains a three-carbon chain ending in the carboxyl group, −COOH. The three-carbon carboxylic acid is propanoic acid, not propanol.

B: soaps and glycerol

Alkaline hydrolysis breaks the ester bonds in fats and oils. It produces glycerol and the sodium or potassium salts of fatty acids, which are soaps.

A: R–C(=O)–OH

A carboxylic acid contains the −C(=O)OH group. A has this group; B is an ester, D an acid anhydride, and E an aldehyde. Sulfuric acid in C is not a carboxylic acid.

D: ₦66.67

Charge per gram is proportional to ionic charge divided by relative atomic mass. For 1 g Mg it is 2/24 mol of electrons; for 10 g Al it is 10×3/27 mol. The charge ratio is (30/27)/(2/24) = 13⅓. At the same cost per unit charge, the cost is ₦5×13⅓ = ₦66.67.

D: heated phosphorus

Heated phosphorus removes oxygen by forming phosphorus oxide, leaving nitrogen. Magnesium is unsuitable because hot magnesium reacts with nitrogen as well as oxygen.

C: forms an insoluble scum with soap

Calcium and magnesium ions in hard water react with soap ions to form insoluble salts, seen as scum. This also makes it harder to obtain a lather.

A: Zn²⁺

Zinc ions form white Zn(OH)₂ with either reagent. The precipitate dissolves in excess NaOH because it is amphoteric, and in excess ammonia because zinc forms a soluble ammine complex. These two observations identify Zn²⁺.