Chemistry 9701 · AS & A Level · Nitrogen and sulfur

Nitrogen and sulfur — practice question

A fertiliser sample was stated to contain only ammonium sulfate, $(\text{NH}_4)_2\text{SO}_4$, together with sand. A $2.96\,\text{g}$ portion of the solid fertiliser was warmed with $40.0\,\text{cm}^3$ of excess $\text{NaOH(aq)}$, and every trace of ammonia formed was boiled off. After the mixture had cooled, the leftover $\text{NaOH(aq)}$ was exactly neutralised by $29.5\,\text{cm}^3$ of $2.00\,\text{mol dm}^{-3}$ $\text{HCl}$. In another experiment, $40.0\,\text{cm}^3$ of the original $\text{NaOH(aq)}$ was exactly neutralised by $39.2\,\text{cm}^3$ of the $2.00\,\text{mol dm}^{-3}$ $\text{HCl}$.
(a(i))[2]

Give balanced equations for these reactions: NaOH with HCl; $(\text{NH}_4)_2\text{SO}_4$ with NaOH.

(a(ii))[1]

Calculate the amount, in moles, of NaOH in the $40.0\,\text{cm}^3$ portion of the original $\text{NaOH(aq)}$ that was neutralised by $39.2\,\text{cm}^3$ of $2.00\,\text{mol dm}^{-3}$ $\text{HCl}$.

(a(iii))[1]

Calculate the amount, in moles, of NaOH in the $40.0\,\text{cm}^3$ of $\text{NaOH(aq)}$ that was left after boiling off the $(\text{NH}_4)_2\text{SO}_4$.

(a(iv))[1]

Using your answers to (ii) and (iii), find the amount, in moles, of NaOH that reacted with the $(\text{NH}_4)_2\text{SO}_4$.

(a(v))[1]

Using your answers to (i) and (iv), calculate the amount, in moles, of $(\text{NH}_4)_2\text{SO}_4$ that reacted with the NaOH.

(a(vi))[1]

Hence work out the mass of $(\text{NH}_4)_2\text{SO}_4$ that reacted.

(a(vii))[1]

Using your answer to (vi), calculate the percentage by mass of $(\text{NH}_4)_2\text{SO}_4$ in the fertiliser. Give your answer to an appropriate number of significant figures.

(b)[2]

Heavy use of nitrogenous fertilisers can harm lakes and streams. This is called eutrophication. What processes take place when excessive amounts of nitrogenous fertilisers enter lakes and streams?

(c)[1]

Ammonia is produced in large quantities by the Haber process. Some of this ammonia is not used to make fertilisers. State one large-scale use of ammonia, other than in the production of nitrogenous fertilisers.

Worked solution & mark scheme

This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Balanced neutralisation equation: $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ (ionic equations permitted).” …

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