JAMB Chemistry 1987

B: Seawater

Seawater contains water together with dissolved salts and other substances. Its components are not combined in one fixed chemical ratio, so it is a mixture. Sugar and sodium chloride are compounds; iron is an element.

C: 0.413 atm

For an ideal gas at constant volume and temperature, P = nRT/V, so pressure is proportional to the amount of gas. Halving the amount halves the pressure: 0.825/2 = 0.4125 atm, which rounds to 0.413 atm.

D: Ethanal

At equal temperature and pressure, vapour density is proportional to molar mass. Ethanal, C₂H₄O, has molar mass 44; ethanoic acid and propanol each have 60, and dichloromethane has 85. Ethanal therefore has the lowest vapour density.

A: Ionic

Atomic numbers 12 and 9 identify magnesium and fluorine. Magnesium loses two electrons to form Mg²⁺, while two fluorine atoms each gain one electron to form F⁻. Their electrostatic attraction forms ionic MgF₂.

B: 16.2

Relative atomic mass is the abundance-weighted mean of isotopic masses: (0.90 × 16) + (0.10 × 18) = 16.2.

B: The bond polarity increases

A larger electronegativity difference causes the bonding electrons to be shared less equally. The resulting partial charges are larger, so the bond is more polar.

D: To coagulate fine mud particles

Alum helps fine suspended particles clump together into larger flocs that can settle or be filtered out. This coagulation step removes suspended matter; it is distinct from disinfection.

D: 13.7%

Na₃AsO₄ has molar mass 3×23 + 75 + 4×16 = 208 g mol⁻¹. The hydrate has mass 208 + 12×18 = 424 g mol⁻¹. Thus 38.9 g hydrate contains 38.9×208/424 = 19.08 g Na₃AsO₄. The solution mass is 100 + 38.9 = 138.9 g, giving 19.08/138.9×100 = 13.7%.

A: Adding NaHCO₃: gas evolves; no gas evolves

Lime juice contains acids that react with bicarbonate to release CO₂. Ethanol is not acidic enough to release CO₂ from sodium bicarbonate. Thus gas forms with lime juice but not with ethanol.

A: 0.094 mol dm⁻³

HCl and NaOH react in a 1:1 mole ratio. At equivalence, c(HCl) = 0.15 × 24.83/39.45 = 0.09441 mol dm⁻³, corresponding to 0.094 mol dm⁻³.

C: 2.24 g

The time is 2.5×3600 = 9000 s, so charge is 2×9000 = 18000 C. Mg²⁺ requires two electrons, giving mass = 18000×24/(2×96500) = 2.238 g, or 2.24 g.

C: 1.2 × 10²⁴

Copper in CuO has oxidation state +2 and gains two electrons to become Cu. One mole of copper therefore requires two moles of electrons: 2×6.0×10²³ = 1.2×10²⁴ electrons.

B: +25.6 kJ mol⁻¹

The molar mass of NH₄NO₃ is 80 g mol⁻¹, so 5 g is 0.0625 mol. Cooling the surroundings means the dissolution absorbs heat. Its enthalpy is +1.6/0.0625 = +25.6 kJ mol⁻¹.

B: −130 kJ mol⁻¹

By Hess’s law, reaction enthalpy equals product formation enthalpy minus the total for reactants: −811 − [(−395) + (−286)] = −130 kJ mol⁻¹. The negative sign means heat is released.

C: Decreasing the reaction temperature

The forward reaction is exothermic. Lowering the equilibrium temperature favours the heat-producing direction, increasing the equilibrium yield of SO₃. A catalyst speeds approach to equilibrium without changing the equilibrium yield.

B: 3Cl₂(g) + 6OH⁻(aq) → ClO₃⁻(aq) + 5Cl⁻(aq) + 3H₂O(l)

Hot concentrated alkali converts chlorine into chloride and chlorate(V): 3Cl₂ + 6OH⁻ → ClO₃⁻ + 5Cl⁻ + 3H₂O. Chlorine is both oxidised to +5 and reduced to −1, so the reaction is disproportionation.

A: Nitrogen

Magnesium reacts with nitrogen to form Mg₃N₂. Water hydrolyses this nitride: Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃. The ammonia gives dense white NH₄Cl fumes with HCl, identifying the original gas as nitrogen.

A: Plenty of cool running water

Immediately flush the affected skin with plenty of running water and remove contaminated clothing while rinsing. Alert the teacher and obtain medical attention. Do not try to neutralise the acid on the skin with an alkali; the reaction can add heat and worsen the injury.

D: Oxygen and sulfur

Oxygen has the allotropes O₂ and O₃. Sulfur also has distinct structural forms, including rhombic and monoclinic sulfur. Thus both elements in the oxygen–sulfur pair exhibit allotropy under ordinary conditions.

C: (ii) and (iv)

Ammonia and hydrogen chloride are highly soluble in water. When either dissolves rapidly inside the flask, the pressure falls and outside air pressure pushes water in, producing a fountain. Nitrogen and nitrous oxide are much less suitable.

C: Passing it directly through CaO

Heating an ammonium salt with calcium hydroxide releases ammonia. It can be dried by passing it directly through calcium oxide. Water absorbs ammonia, while concentrated sulfuric acid and calcium chloride react with it.

C: Iron, carbon and chromium

Stainless steel is an iron-based alloy containing carbon and chromium. Chromium forms a protective oxide layer that improves corrosion resistance. Some grades contain additional elements such as nickel.

D: Cooling a molten mixture of the required elements

A common way to make an alloy is to mix the constituent elements in a molten metallic mixture and then allow it to solidify. Mixing them before solidification distributes the components through the alloy.

D: All metals

All metals can suffer corrosion in a suitable environment, although their resistance differs greatly. Even noble metals can be attacked under particular conditions. Rusting is the specific corrosion of iron, not the name for all metal corrosion.

C: The 2s and 2p orbitals hybridise

Carbon has four valence electrons. In the valence-bond description of methane, one 2s orbital and three 2p orbitals mix to form four sp³ hybrid orbitals, each used in a C–H bond. This accounts for four equivalent tetrahedral bonds.

A: Branched-chain alkanes

Branched alkanes generally resist premature autoignition better than corresponding straight-chain alkanes. This increases knock resistance, allowing a petrol engine to operate more effectively at high compression.

B: Triiodomethane

Ethanol gives the iodoform reaction with alkaline iodine/hypoiodite. The yellow precipitate is CHI₃, called triiodomethane or iodoform. The other listed compounds are not the yellow solid formed in this test.

D: Ethane, methane and propane

Methane, ethane and propane are small hydrocarbons with very low boiling points. They belong to the light gaseous fraction. Petrol, kerosene and benzene boil at higher temperatures and are not all part of the most volatile fraction.

C: Alkali

The aqueous ash extract supplies alkaline substances. These react with fats in palm oil during saponification to form fatty-acid salts, which are soap. The palm oil supplies the fat, while the ash supplies the alkali.

A: 2-Methylbuta-1,3-diene

2-Methylbuta-1,3-diene is isoprene, a conjugated diene. Its molecules can polymerise into polyisoprene, a rubber. The conjugated double bonds make it a suitable rubber monomer.

D: Propanoic acid

Oxidation of a primary alcohol first gives an aldehyde and then a carboxylic acid when oxidation continues. Propan-1-ol therefore gives propanoic acid, retaining its three-carbon chain.