Chemistry 2012 Jamb Examination

Lead(II) ethanoate reacts with hydrogen sulphide gas to form black lead(II) sulphide. This reaction is a classical test for detecting the presence of hydrogen sulphide. When the gas comes into contact with the moist filter paper containing lead(II) ions, the following reaction occurs:

Pb(CH₃COO)₂ + H₂S → PbS + 2CH₃COOH

The black colour of lead(II) sulphide confirms the presence of hydrogen sulphide gas. Other gases listed do not produce this characteristic black precipitate with lead(II) ethanoate.

At standard temperature and pressure (s.t.p), one mole of any gas occupies 22.4 dm³. The volume of 0.05 moles of carbon dioxide is calculated as:
\[ \text{Volume} = 0.05 \times 22.4 = 1.12\, \text{dm}^3 \]
This shows that 0.05 moles of carbon dioxide gas will occupy a volume of 1.12 dm³ at s.t.p.

In sodium borohydride (NaBH₄), sodium has an oxidation number of +1 and hydrogen has -1. Let the oxidation number of boron be \( x \). The sum of oxidation numbers in a neutral compound equals zero:
\[ +1 + x + 4(-1) = 0 \]\[ +1 + x - 4 = 0 \]\[ x = +3 \]
Therefore, boron has an oxidation number of +3 in NaBH₄.

In this reaction, sodium peroxide (Na₂O₂) reacts with hydrogen peroxide (H₂O₂) to form sodium hydroxide and oxygen gas. Oxygen in Na₂O₂ has an oxidation state of -1, while in O₂ it is 0. The increase in oxidation number from -1 to 0 indicates oxidation. Therefore, sodium peroxide is oxidised while hydrogen peroxide acts as the oxidising agent.

For copper deposition, the reaction at the cathode is:
\[ \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \]
This means 2 Faradays (2 × 96500 C) deposit 1 mole of Cu. The number of moles of Cu deposited by 360 C is:
\[ n = \dfrac{360}{2 \times 96500} \]\[ n = 1.866 \times 10^{-3} \; \text{mol} \]
Hence, 1.87 × 10⁻³ mol of Cu²⁺ will be deposited.

To compare solubilities of different solutes in the same solvent, their solubility curves are plotted on the same axes. This allows direct comparison of how solubility changes with temperature. When all curves are on one graph, differences and similarities in solubility behaviour become clear and measurable.

Molar mass of KClO₃ = 39 + 35.5 + (16 × 3) = 122.5 g mol⁻¹.

Decrease in solubility = 1.5 - 0.5 = 1.0 mol dm⁻³.

This means 1.0 mol of KClO₃ crystallised out per dm³ of solution.

Mass crystallised = 1.0 × 122.5 = 122.5 g.

Therefore, 122.5 g of KClO₃ was crystallised out.

Carbon(II) oxide, or carbon monoxide, binds strongly with haemoglobin to form carboxyhaemoglobin, reducing oxygen transport in the blood. Prolonged exposure leads to oxygen deprivation in the brain, causing brain damage or death in severe cases. Other pollutants listed do not directly interfere with oxygen transport to this extent.

The titration between a strong acid and a weak base yields an acidic salt, leading to a solution with pH less than 7 at the equivalence point. Hydrochloric acid (strong acid) reacting with magnesium hydroxide (weak base) produces such an acidic solution due to hydrolysis of the salt formed.

The basicity of an acid refers to the number of hydrogen ions (H⁺) or hydroxonium ions (H₃O⁺) produced per molecule of the acid when dissolved in water. For example, H₂SO₄ produces two H₃O⁺ ions per molecule, hence it is dibasic. Acid strength, pH, and concentration relate to other properties, not the number of ions produced.