JAMB Chemistry 2020 Paper

Concept:The electrolysis of dilute sodium chloride solution liberates oxygen gas at the anode, and the quantity of electricity needed depends on the mole ratio of electrons to oxygen molecules.Explanation:At the anode, hydroxide ions are discharged in preference to chloride ions.The anode half-reaction is:$$4\mathrm{OH}^- \rightarrow 2\mathrm{H}_2\mathrm{O} + \mathrm{O}_2 + 4e^-$$From this balanced equation, $1$ mole of $\mathrm{O}_2$ molecules is produced by $4$ moles of electrons.The charge carried by $1$ mole of electrons is $1$ Faraday.So, $1$ Faraday $= 96{,}500$ C.Therefore, $1$ mole of $\mathrm{O}_2$ requires $4 \times 96{,}500$ C of electricity.For $0.125$ mole of $\mathrm{O}_2$:Quantity of electricity $= 0.125 \times 4 \times 96{,}500$ CQuantity of electricity $= 48{,}250$ C.Answer:The quantity of electricity required is $48{,}250$ C, which is option B.