28 reviewed questions with answers and explanations.
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The addition of water to calcium oxide leads to
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.
AI-reviewed · calculation checkedC₃H₈(g) + 5O₂(g) → 4H₂O(g) + 3CO₂(g). What volume of oxygen at STP is needed to burn 50 cm³ of propane measured at STP?
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.
AI-reviewed · calculation checked30 cm³ of hydrogen is collected over water at 27 °C and total pressure 780 mmHg. Taking the given water-vapour pressure as 10 mmHg, calculate the dry hydrogen volume at 760 mmHg and 7 °C. Use T(K) = t(°C) + 273.
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.
AI-reviewed · calculation checkedSulfur dioxide and methane are released simultaneously at opposite ends of a narrow tube under the same conditions. What is the ratio R(SO₂):R(CH₄)? [S = 32, O = 16, C = 12, H = 1]
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.
AI-reviewed · calculation checkedAn atom is shown with four occupied electron shells and two electrons in the outermost shell. When it combines with chlorine, which type of bond is formed?
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.
AI-reviewed · source checkedThe lines in the hydrogen emission spectrum result from emission of
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.
AI-reviewed · source checkedWhich property is used to separate nitrogen and oxygen by industrial fractional distillation of liquid air?
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.
AI-reviewed · source checkedExcess phosphorus is burned in a gas jar of air. The residual gas passes through concentrated KOH solution and then concentrated H₂SO₄. Which gases are collected?
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.
AI-reviewed · source checkedSoap lather is a colloid in which
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.
AI-reviewed · source checkedAt 25 °C, what is the pH after mixing 100 cm³ of 0.1 mol dm⁻³ HCl with 100 cm³ of 0.2 mol dm⁻³ NaOH, assuming additive volumes?
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.
AI-reviewed · calculation checkedWhat volume of 0.1 mol dm⁻³ sulfuric acid is needed to react completely with 2.86 g of sodium carbonate decahydrate, Na₂CO₃·10H₂O? [Na = 23, C = 12, H = 1, O = 16]
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.
AI-reviewed · calculation checkedWhat mass of gold is deposited from gold(III) sulfate when 15 A passes for 193 s, assuming full current efficiency? [Au = 197; F = 96,500 C mol⁻¹]
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.
AI-reviewed · calculation checkedFe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s). Which statement follows?
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.
AI-reviewed · calculation checkedIn 2FeCl₂(s) + Cl₂(g) → 2FeCl₃(s), which is the reducing 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.
AI-reviewed · calculation checkedAn energy diagram for P₂ + Q₂ → 2PQ places reactants at 10 kJ, products at 15 kJ and the peak at 40 kJ. What is the activation energy for the backward reaction?
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.
AI-reviewed · calculation checkedFor 2X(g) + Y(g) → Z(g), the rate of formation of Z is independent of [Y] and quadruples when [X] doubles. Which rate law agrees?
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.
AI-reviewed · calculation checked2Cl₂(g) + 2H₂O(g) ⇌ 4HCl(g) + O₂(g), ΔH° = +115 kJ mol⁻¹. How does decreasing temperature affect the equilibrium?
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.
AI-reviewed · source checkedConsider: (i) 3CuO(s) + 2NH₃(g) → 3Cu(s) + 3H₂O(l) + N₂(g); (ii) 2NH₃(g) + 3Cl₂(g) → 6HCl(g) + N₂(g); (iii) 4NH₃(g) + 3O₂(g) → 6H₂O(l) + 2N₂(g). Which property of ammonia do these reactions demonstrate?
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.
AI-reviewed · calculation checkedA heated flask contains solution X. A delivery tube carries the gas from the flask into an inverted water-filled receiver, where nitrogen gas (N₂) is collected over water. Which mixture could X contain?
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.
AI-reviewed · source checkedWhat is observed when aqueous iron(III) chloride is mixed with aqueous sodium hydroxide?
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.
AI-reviewed · source checkedWhy are alloys often preferred to pure metals for practical applications?
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.
AI-reviewed · source checkedWhat is the IUPAC name of CH₃CH₂CH(OH)CH(CH₃)₂?
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.
AI-reviewed · calculation checkedEthene undergoes addition polymerisation with oxygen used as an initiator: nCH₂=CH₂ → [–CH₂–CH₂–]ₙ. Which material is produced?
B: Polythene
Ethene monomers join by addition polymerisation. Their double bonds open to form the repeating unit –CH₂–CH₂–, giving polyethene, commonly called polythene.
AI-reviewed · calculation checkedWhat are the products of complete combustion of a pure hydrocarbon in excess oxygen?
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.
AI-reviewed · source checkedCracking medicinal paraffin produces a gas that gives a pop with a lighted splint and an oily liquid that decolourises bromine solution. Which listed pair matches these observations?
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.
AI-reviewed · source checkedWhich formula represents an aromatic compound?
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.
AI-reviewed · source checkedTerylene is formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid by which type of polymer-forming reaction?
B: Condensation
The diol and dicarboxylic acid form ester links while eliminating small water molecules. Repeated condensation produces the polyester Terylene.
AI-reviewed · source checkedWhich statement is true of benzene and hexane under suitable reaction conditions?
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.
AI-reviewed · source checked