JAMB Chemistry 1985

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.