WAEC Chemistry 2013 Paper

Concept:Salt hydrolysis determines whether a salt solution is acidic, basic, or neutral.Explanation:$\mathrm{Na_2CO_3}$ is formed from a weak acid, $\mathrm{H_2CO_3}$, and a strong base, $\mathrm{NaOH}$.So, its aqueous solution is basic.The carbonate ion undergoes hydrolysis as follows:$$\mathrm{CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-}$$The production of $\mathrm{OH^-}$ ions makes the solution basic, giving $\mathrm{pH > 7}$.In contrast, $\mathrm{FeCl_3}$ and $\mathrm{CuSO_4}$ are salts of strong acids and weak bases, so their solutions are acidic.$\mathrm{KNO_3}$ is a salt of a strong acid and a strong base, giving a neutral solution.Therefore, only $\mathrm{Na_2CO_3}$ has $\mathrm{pH > 7}$.Answer:D. $\mathrm{Na_2CO_3(aq)}$