JAMB Chemistry 2018 Paper

Concept:Electrolysis of aqueous $\mathrm{CuSO}_4$ using inert platinum electrodes means water also takes part in the discharge of ions, so the anode region becomes acidic and oxygen gas is given off.Explanation:The solution contains $\mathrm{Cu}^{2+}$, $\mathrm{H}^+$, $\mathrm{SO}_4^{2-}$, and $\mathrm{OH}^-$ ions.At the cathode, $\mathrm{Cu}^{2+}$ is preferentially discharged because copper is below hydrogen in the electrochemical series, so copper metal is deposited.At the anode, $\mathrm{OH}^-$ is discharged in preference to $\mathrm{SO}_4^{2-}$.The half-reaction is: $4\mathrm{OH}^- \rightarrow 2\mathrm{H}_2\mathrm{O} + \mathrm{O}_2 + 4e^-$.Therefore, oxygen gas is evolved in the anode compartment.The $\mathrm{H}^+$ and $\mathrm{SO}_4^{2-}$ ions left behind combine to form sulfuric acid, making the solution acidic.Since acids turn blue litmus red, the litmus solution added to the anode compartment will turn red.Answer:D. The litmus turns red and oxygen is evolved.