Chemistry 9701 · AS & A Level · Nitrogen and sulfur

Nitrogen and sulfur — practice question

A hydrated double salt sample, $\mathrm{Cu(NH_4)}_x(\mathrm{SO}_4)_2\cdot6\mathrm{H}_2\mathrm{O}$, was heated with an excess of sodium hydroxide. Ammonia was released. The ammonia formed was absorbed in 40.0 cm$^3$ of 0.400 mol dm$^{-3}$ hydrochloric acid. The solution that remained then needed 25 cm$^3$ of 0.12 mol dm$^{-3}$ sodium hydroxide to neutralise the extra acid.
(a)[1]

Write the ionic equation for the reaction of ammonium ions with hydroxide ions.

(b(i))[1]

Calculate the amount, in moles, of hydrochloric acid present in 40.0 cm$^3$ of 0.400 mol dm$^{-3}$ solution.

(b(ii))[1]

Calculate the amount, in moles, of sodium hydroxide needed to neutralise the extra acid. This will match the amount of hydrochloric acid remaining in excess.

(b(iii))[1]

Calculate the amount, in moles, of hydrochloric acid that reacted with ammonia.

(b(iv))[1]

Calculate the amount, in moles, of ammonium ions in the double salt sample.

(b(v))[2]

The sample had 0.413 g of copper. Use this together with your answer to (iv) to find the value of $x$ in $\mathrm{Cu(NH_4)}_x(\mathrm{SO}_4)_2\cdot6\mathrm{H}_2\mathrm{O}$.

(b(vi))[1]

Calculate the $M_r$ of $\mathrm{Cu(NH_4)}_x(\mathrm{SO}_4)_2\cdot6\mathrm{H}_2\mathrm{O}$.

Worked solution & mark scheme

This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “$\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}$” …

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