JAMB Chemistry 1991

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.