JAMB Chemistry 1992

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.