JAMB Chemistry 2004

A: A decrease in pressure by expansion

The product side has two moles of gas for each mole of N₂O₄. Decreasing pressure by expansion at fixed temperature favours the side with more gas molecules, producing more NO₂.

C: +7

In neutral HClO₄, hydrogen is +1 and four oxygen atoms total −8. Therefore 1 + x − 8 = 0, so chlorine is +7.

C: HI

At 298.15 K, standard molar gas entropies are approximately HF 173.8, HCl 186.9, HBr 198.7 and HI 206.6 J mol⁻¹ K⁻¹. HI has the largest value in this series.

A: 54.1 g

For Ag⁺ + e⁻ → Ag, one mole of electrons deposits one mole of silver. Q = It = 10 × 4830 = 48,300 C. Mass = 48,300/96,500 × 108 = 54.055 g, or 54.1 g to one decimal place.

B: H₂(g) + I₂(g) ⇌ 2HI(g)

H₂ + I₂ ⇌ 2HI has two moles of gas on each side. Compression at fixed temperature scales numerator and denominator of its pressure quotient equally, so it causes no equilibrium shift in the ideal-gas approximation.

C: Reduced

Zinc is +2 in ZnO and 0 in elemental Zn. The decrease in oxidation number means zinc is reduced; carbon monoxide is oxidised to carbon dioxide.

D: 448 cm³

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O. The carbonate molar mass is 100 g mol⁻¹, so 2 g gives 0.020 mol CO₂. Volume = 0.020 × 22.4 = 0.448 dm³ = 448 cm³.

A: Sublimation

Sublimation is the direct change from solid to gas without passing through the liquid state. Evaporation begins with a liquid, while crystallisation forms a solid.

D: 4.0 g

CuO + H₂SO₄ → CuSO₄ + H₂O is a 1:1 reaction. The acid amount is 4.9/98 = 0.050 mol. The required CuO mass is 0.050 × 80 = 4.0 g.

C: CₙH₂ₙ₊₁

An acyclic saturated alkyl group is formed by removing one hydrogen from an alkane, CₙH₂ₙ₊₂. Its formula is therefore CₙH₂ₙ₊₁–.

D: C₂H₄

Concentrated sulfuric acid at about 180 °C dehydrates ethanol: C₂H₅OH → C₂H₄ + H₂O. The product is ethene.

C: Decrease the soap’s solubility

Concentrated sodium chloride lowers the solubility of sodium soaps in the aqueous mixture, allowing soap to separate. This separation is called salting out.

A: Releases a large amount of heat during combustion

Acetylene burns in oxygen with a concentrated, high-temperature flame. The heat can melt the metal locally so a weld can form.

C: 2-Methylbutane

2-Methylbutane has five carbon atoms and molecular formula C₅H₁₂. Butane and 2-methylpropane have only four carbons; the structure labelled 2-ethylbutane has six.

B: Hydrolysis

In the reverse direction, an ester reacts with water to form an alcohol and a carboxylic acid. This is ester hydrolysis. Alkaline hydrolysis producing a carboxylate salt is called saponification.

C: Decarboxylation

The products contain carbon dioxide and a hydrocarbon with one fewer carbon atom than the acid. Removal of the carboxyl group as CO₂ is decarboxylation.

C: High acidity

Acidic conditions often increase dissolution and mobility of metal-containing minerals. This can allow metal ions to leach through soil and contaminate groundwater.

A: Na₂CO₃

A normal salt contains no replaceable acidic hydrogen from incomplete neutralisation. Na₂CO₃ is the fully neutralised sodium salt of carbonic acid; NaHCO₃, NaHSO₄ and NaHS are acid salts. Normal salt does not mean its solution has neutral pH.

D: 2.0 cm³

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. NaOH amount = 0.020 × 0.1 = 0.0020 mol, requiring 0.0010 mol acid. Volume = 0.0010/0.5 = 0.0020 dm³ = 2.0 cm³.

B: Solution

The dissolved portion of calcium sulfate exists as dispersed ions, forming a true solution. Its low solubility limits how much dissolves; it does not make the dissolved portion a colloid.

C: Have weak attractions relative to their kinetic energy

In a dilute gas, intermolecular attractions are weak compared with molecular kinetic energy. The molecules move freely and do not maintain the regular fixed arrangement found in a crystal.

D: Spherical

An s orbital has spherical symmetry: its probability density at a given distance from the nucleus is independent of direction.

C: Neon and hydrogen

Hydrogen is combustible and can react with oxygen supplied by air. Neon is inert and acts as a diluent. The other listed mixtures contain no readily combustible gas.

A: Electronegativity

Chlorine attracts bonding electrons more strongly than hydrogen, making H–Cl polar. In Cl–Cl the identical atoms share equally. The relevant property is electronegativity; molecular HCl is polar covalent.

A: 8 valence electrons

A halogen atom has seven valence electrons. Gaining one electron forms a halide ion with a completed outer-shell octet.

B: −273 °C

Extrapolating the ideal-gas Charles-law line gives zero volume at 0 K, approximately −273 °C. Real gases condense before reaching this extrapolated limit, so it is not a claim that a real gas physically occupies zero volume.

A: Hydrogen and carbon monoxide

Steam reacts with red-hot carbon: C + H₂O(g) → CO + H₂. The mixture of carbon monoxide and hydrogen is called water gas.

D: Mordant

A mordant helps fix a dye to a fibre. Aluminium hydroxide can bind dye molecules and support their attachment to the material.

B: d electrons as well as outer s electrons

The outer s and nearby d electrons of transition metals can participate in bonding or ion formation. Their relatively similar energies allow different numbers of electrons to be involved, producing several oxidation states.

D: Charcoal

Porous charcoal adsorbs coloured substances onto its large internal surface. Activated charcoal is therefore useful for removing colour from solutions such as sugar solutions.

A: Cooling a molten mixture of the metals

A common alloy-making method melts and mixes the constituent metals, then allows the mixture to solidify. This distributes the constituents through the resulting metallic material.

B: Reduction

Sulfur dioxide acts as a reducing bleaching agent, converting susceptible coloured substances into colourless forms. In some materials the colour can return when air reoxidises the reduced substance.