(a)[2]
Explain what the term buffer solution means.
(b(i))[1]
Write an expression for the acid dissociation constant, $K_a$, for ammonium ions, $\text{NH}_4^+(aq)$.
(b(ii))[2]
Write two equations to show how a solution containing ammonium ions, $\text{NH}_4^+(aq)$, and ammonia, $\text{NH}_3(aq)$, may function as a buffer.
(b(iii))[3]
Calculate the pH of the buffer solution formed at $298\ \text{K}$. The value of $K_a$ for $\text{NH}_4^+(aq)$ is $5.6 \times 10^{-10}$ at $298\ \text{K}$. A buffer solution was prepared by adding $0.80\ \text{dm}^3$ of $0.25\ \text{mol dm}^{-3}$ ammonia, an excess, to $0.20\ \text{dm}^3$ of $0.20\ \text{mol dm}^{-3}$ hydrochloric acid.