2018 H2 Chem P1 Q15 — Acid base equilibrium Solution

The answer is A (ammonia and ammonium chloride).
Keeping the pH steady at about 10 needs a buffer: a weak base with its conjugate acid (or a weak acid with its conjugate base). \(\text{NH}_3\) is a weak base and \(\text{NH}_4^+\) (from \(\text{NH}_4\text{Cl}\)) is its conjugate acid, with \(\text{p}K_\text{a}(\text{NH}_4^+) = 9.25\), so this basic buffer holds the pH at about 9 to 10:
\(\text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq})\)
Added \(\text{H}^+\) is removed by the large reservoir of \(\text{NH}_3\): \(\text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+\). Added \(\text{OH}^-\) is removed by the large reservoir of \(\text{NH}_4^+\): \(\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}\).
• B is wrong: both substances are bases. There is no conjugate acid to remove added \(\text{OH}^-\), so this is not a buffer, and the strong base NaOH makes the pH far above 10.
• C is wrong: ethanoate ion is the conjugate base of ethanoic acid, but no ethanoic acid is present, so there is no buffer pair; the strong base NaOH again makes the pH far above 10.
• D is wrong: NaOH alone is a fully ionised strong base with no acid component, so it cannot resist changes in pH, and its solution has a pH well above 10.