Edexcel IAL Chemistry 14.18 Action of a buffer solution

An answer that makes reference to the following points:

- the mixture contains a large amount/ (large) reservoir of both ammonium ions and ammonia (1) / of $\mathrm{NH}_{4}^{+}$and $\mathrm{NH}_{3}$

Either

- added $\mathrm{H}^{+}$reacts with ammonia to form ammonium ions $/ \mathrm{H}^{+}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{NH}_{4}^{+}$

Or

- added $\mathrm{H}^{+}$combines with $\mathrm{OH}^{-}$ions in water to form water $/ \mathrm{H}^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{H}_{2} \mathrm{O}$

And ammonia reacts with water to produce $\mathrm{OH}^{-} \quad$ (1) ions $/ \mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{OH}^{-}$

- ratio of ammonium ions to ammonia hardly (1) changes

Do not award incorrect formulae such as $\mathrm{NH}_{3}{ }^{-}$in M1 and M2 but allow TE in M3 Ignore comments about acid / base in relation to $\mathrm{NH}_{4}^{+}$/ $\mathrm{NH}_{3}$ unless defined

Allow → instead of ⇌ Allow $\mathrm{H}_{3} \mathrm{O}^{+}$

Allow → instead of ⇌

This marking point must include at least one ionic equation

Allow remains constant

Allow pH is unchanged / changes very little because added $\mathrm{H}^{+}$removed and change in concentration of $\mathrm{NH}_{3}$ and $\mathrm{NH}_{4}^{+}$is small