JAMB Chemistry 2016 Paper

Concept:Electrolysis of acidified water releases oxygen at the anode; the volume of gas is found from the charge passed and Faraday's law.Explanation:First calculate the total charge passed:$$Q = I \times t = 5\, \mathrm{A} \times 193\, \mathrm{s} = 965\, \mathrm{C}$$At the anode, water is oxidised to oxygen gas:$$2\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{O}_2 + 4\mathrm{H}^+ + 4e^-$$Thus 4 moles of electrons produce 1 mole of oxygen molecules.The charge needed for 4 moles of electrons is:$$4 \times 96500\, \mathrm{C\, mol^{-1}} = 386000\, \mathrm{C}$$So the volume of oxygen at s.t.p. is given by:$$V = \frac{Q}{4F} \times 22.4 \, \mathrm{dm}^3 = \frac{965}{4 \times 96500} \times 22.4$$$$V = 0.056\, \mathrm{dm}^3 = 56\, \mathrm{cm}^3$$Answer:The volume of oxygen evolved is $0.056\, \mathrm{dm}^3$ (Option A).