JAMB Chemistry 1983

C: NaHSO₄

Sodium hydrogensulfate, NaHSO₄, forms an acidic solution. Its hydrogen ions react with carbonate to release CO₂, which makes limewater milky. Its sulfate ions form insoluble white BaSO₄ with barium ions.

B: glycerol

Alkaline hydrolysis of fats or oils produces the salts of fatty acids (soap) and glycerol. Glycerol is an alcohol with three hydroxyl groups.

D: oxy-acetylene flame

Burning acetylene in oxygen produces the hot oxy-acetylene flame used for welding and cutting suitable metals. Oxygen alone is not a fuel.

B: CH₂

Alkanes have the general formula CₙH₂ₙ₊₂. Increasing n by one adds one carbon atom and two hydrogen atoms, so successive members differ by CH₂.

D: a p-block element

The final electrons occupy a p subshell (3p²), so the element belongs to the p block. The configuration contains 14 electrons and corresponds to silicon.

D: an ester

Yeast converts glucose to ethanol, so Y is ethanol. Ethanol reacts with ethanoic acid to form ethyl ethanoate and water. Ethyl ethanoate belongs to the ester family, making W an ester.

B: C₆H₁₂O₆

The atomic ratio gives empirical formula CH₂O, with formula mass 12 + 2 + 16 = 30. The molecular mass is 180, so the multiplier is 180 ÷ 30 = 6. The molecular formula is C₆H₁₂O₆.

C: brewing industry

Yeast ferments glucose without air to form ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. This process is central to brewing.

B: P ∝ T/V

For a fixed amount of ideal gas, PV = nRT. Rearranging gives P = nR(T/V). Since nR is constant, P is proportional to T/V, with T measured in kelvin.

D: It turns blue

Heating produces ammonia and hydrogen chloride. Ammonia has the lower molar mass and passes through the porous plug faster. It reaches the damp paper first and produces an alkaline response, turning the paper blue.

C: brick-red

Calcium produces a brick-red (orange-red) flame. Sodium gives yellow, potassium lilac, and copper a blue-green flame.

D: 1 and 3

Oxidation means an increase in oxidation state. Iron changes from 0 to +2 in reaction 1 and from +2 to +3 in reaction 3. It remains +2 in reaction 2 and is reduced from +3 to +2 in reaction 4. Therefore, iron is oxidised in 1 and 3.

D: 2.16 g

The copper amount is 0.63/63 = 0.010 mol. Cu²⁺ needs two electrons per atom, so 0.020 mol of electrons passes through both cells. Ag⁺ needs one electron per atom, giving 0.020 × 108 = 2.16 g of silver.

E: iron is higher in the electrochemical series than copper

Iron is more reactive than copper and more readily loses electrons. Fe is oxidised to Fe²⁺ while Cu²⁺ gains those electrons and becomes copper metal.

A: 2-methylbut-1-ene

Choose the longest chain containing the double bond: it has four carbon atoms. Number from the double-bond end, giving but-1-ene. The CH₃ branch is on carbon 2, so the name is 2-methylbut-1-ene.

A: Efflorescence

Efflorescence is the loss of water of crystallisation from a hydrated salt on exposure to air. It differs from deliquescence, in which a solid absorbs enough moisture to dissolve.

E: Chloride ion is oxidized.

At the anode, chloride ions lose electrons: 2Cl⁻ → Cl₂ + 2e⁻. Loss of electrons is oxidation. At the cathode, sodium ions gain electrons to form sodium metal.

D: spraying the water with detergents

Detergents contain surfactants that help break oil into small droplets dispersed in water. Dispersion does not destroy the oil; real spill treatment uses selected dispersants and considers environmental conditions.

E: Z is the least acidic

Higher pH corresponds to lower hydrogen-ion concentration. Z has the highest pH, 9.0, and is therefore the least acidic. X, with pH 3.0, is more acidic than Y, with pH 5.0.

B: carbon, iron and chromium

Stainless steel is an iron alloy containing chromium and usually some carbon. Chromium forms a protective surface oxide that improves corrosion resistance.

A: 2.0 cm³

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. The NaOH amount is 0.1 × 20/1000 = 0.002 mol. Half as many moles of H₂SO₄ are needed: 0.001 mol. Its volume is 0.001/0.50 = 0.002 dm³ = 2.0 cm³.

A: Some CuO was reacted with excess dilute H₂SO₄

Copper(II) nitrate requires nitrate ions. CuO reacts with nitric acid to form Cu(NO₃)₂ and water. Sulfuric acid would instead produce copper(II) sulfate, so step A cannot make the requested salt.

C: 2.5 mol dm⁻³

The formula mass of KClO₃ is 39 + 35.5 + 3×16 = 122.5. The dissolved amount is 3.06/122.5 ≈ 0.0250 mol in 10 cm³ of water. Per dm³ of water, this is 0.0250×100 = 2.50 mol.

C: Yields more engine fuels

Cracking splits larger hydrocarbon molecules into smaller alkanes and alkenes. This increases the supply of shorter hydrocarbons useful in petrol and other engine fuels.

B: CuSO₄

CuSO₄ contains sulfate ions, which give white BaSO₄ with barium chloride. Its copper ions produce a blue-green flame. Ammonium sulfate can give the sulfate precipitate but does not give the copper flame colour.

D: the number of neutrons and protons

Almost all atomic mass comes from the nucleus. Protons and neutrons each have a mass close to 1 u; electrons contribute very little. The total number of protons and neutrons is the mass number.

E: Nitric acid to potassium hydroxide solution

Nitric acid supplies H⁺ ions and potassium hydroxide supplies OH⁻ ions. They combine to form water: H⁺ + OH⁻ → H₂O. The overall reaction is HNO₃ + KOH → KNO₃ + H₂O, producing potassium nitrate and water.

A: using powdered zinc

Powdered zinc has a larger exposed surface area than the same mass in pellets. More zinc is in contact with the acid at once, so more successful collisions can occur per second.

A: 1.35 g

The reaction is 1:1 in Zn and H₂SO₄. Acid supplied = 1.0×10/1000 = 0.010 mol, which consumes 0.010×65 = 0.65 g of zinc. The zinc remaining is 2.00 − 0.65 = 1.35 g.