Chemistry 9701 · AS & A Level · Nitrogen and sulfur
Nitrogen and sulfur — practice question
Chile saltpetre is a mineral occurring in Chile and Peru, made up mainly of sodium nitrate, $\text{NaNO}_3$. The mineral is purified to raise the proportion of the $\text{NaNO}_3$, which is used as a fertiliser and in some fireworks. To determine the purity of a sodium nitrate sample, the compound is heated in $\text{NaOH(aq)}$ with Devarda's alloy, which contains aluminium. This converts the sodium nitrate into ammonia, which is boiled off and then dissolved in acid. $3\text{NaNO}_3\text{(aq)} + 8\text{Al(s)} + 5\text{NaOH(aq)} + 18\text{H}_2\text{O(l)} \rightarrow 3\text{NH}_3\text{(g)} + 8\text{NaAl(OH)}_4\text{(aq)}$. The ammonia gas formed is absorbed in an excess of $\text{H}_2\text{SO}_4$ of known concentration: $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4$. The unreacted $\text{H}_2\text{SO}_4$ is then determined by back-titration with $\text{NaOH}$ of known concentration: $\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$.
(a(i))
A $1.64\,\text{g}$ sample of impure $\text{NaNO}_3$ was reacted with an excess of Devarda's alloy. The $\text{NH}_3$ produced was absorbed in $25.0\,\text{cm}^3$ of $1.00\,\text{mol dm}^{-3}$ $\text{H}_2\text{SO}_4$. Calculate the amount, in moles, of $\text{H}_2\text{SO}_4$ present in the $25.0\,\text{cm}^3$ of $1.00\,\text{mol dm}^{-3}$ $\text{H}_2\text{SO}_4$.
(a(ii))
Calculate the amount, in moles, of $\text{NaOH}$ present in $16.2\,\text{cm}^3$ of $2.00\,\text{mol dm}^{-3}$ $\text{NaOH}$.
(a(iii))
Use your answer to (ii) to calculate the amount, in moles, of $\text{H}_2\text{SO}_4$ that reacted with $16.2\,\text{cm}^3$ of $2.00\,\text{mol dm}^{-3}$ $\text{NaOH}$.
(a(iv))
Use your answers to (i) and (iii) to calculate the amount, in moles, of $\text{H}_2\text{SO}_4$ that reacted with the $\text{NH}_3$.
(a(v))[1]
Use your answer to (iv) to calculate the amount, in moles, of $\text{NH}_3$ that reacted with the $\text{H}_2\text{SO}_4$.
(a(vi))[1]
Use your answer to (v) to calculate the amount, in moles, of $\text{NaNO}_3$ that reacted with the Devarda's alloy.
(a(vii))[1]
Hence calculate the mass of $\text{NaNO}_3$ that reacted.
(a(viii))[2]
Use your answer to (vii) to calculate the percentage by mass of $\text{NaNO}_3$ in the impure sample. Give your answer to an appropriate number of significant figures.
(b)[1]
The reaction above is a redox reaction. What oxidation numbers does nitrogen have in $\text{NaNO}_3$ and in $\text{NH}_3$?
Worked solution & mark scheme
This 6-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Therefore, $n(\mathrm{H_2SO_4}) = \frac{25.0 \times 1.00}{1000} = 0.025\ \mathrm{mol}$” …