JAMB Chemistry 1999

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.