WAEC Chemistry 2016 Paper

Concept:A salt that forms an acidic solution when dissolved in water will turn blue litmus paper red.Explanation:Ammonium chloride, $\mathrm{NH_4Cl}$, is a salt of a weak base, $\mathrm{NH_4OH}$, and a strong acid, $\mathrm{HCl}$.When it dissolves in water, the ammonium ion undergoes hydrolysis:$$\mathrm{NH_4^+ + H_2O \rightleftharpoons NH_4OH + H^+}$$This reaction produces free hydrogen ions, $\mathrm{H^+}$, making the solution acidic.Blue litmus paper turns red in the presence of an acid.In contrast, $\mathrm{CH_3COONa}$ is a salt of a weak acid and a strong base, so its solution is basic.Both $\mathrm{NaCl}$ and $\mathrm{KCl}$ are salts of strong acids and strong bases, so their solutions are neutral.Therefore, only $\mathrm{NH_4Cl}$ produces the acidic solution described.Answer:B. $\mathrm{NH_4Cl}$