Chemistry - GOC Question with Solution | TestHub

(A) 4-Nitrobenzoic acid ($\mathrm{p}{K}_{\mathrm{a}}$ 3.4) $>$ benzoic acid (4.2) $>$ 4-methoxybenzoic acid (4.5): the $-\mathrm{N}{\mathrm{O}}_2$ stabilises the carboxylate, the +M $-\mathrm{OC}{\mathrm{H}}_3$ destabilises it. Correct.

(B) Each $-\mathrm{N}{\mathrm{O}}_2$ on the ring stabilises the phenoxide ion more: $\mathrm{p}{K}_{\mathrm{a}}$ 0.4, 4.1, 7.2, 10.0 and 10.2 for the five phenols. Correct.

(C) $\mathrm{p}{K}_{\mathrm{a}}$: acetic acid 4.8, carbonic acid 6.4, phenol 10, water 15.7, ethanol 16, ethyne 25. Correct.

(D) The acid strength of the hydrogen halides rises down the group as the $\mathrm{H}-\mathrm{X}$ bond gets weaker: $\mathrm{HI}$ $>$ $\mathrm{HBr}$ $>$ $\mathrm{HCl}$ $>$ $\mathrm{HF}$. The order given is reversed. Wrong.

Answer: (A), (B), (C).