Chemistry 9701 · AS & A Level · Analytical techniques
Analytical techniques — practice question
Spathose is an iron ore containing iron(II) carbonate, $\text{FeCO}_3$. The proportion of iron(II) carbonate in spathose may be found by titration with acidified potassium dichromate(VI) solution, using a suitable indicator. The ionic equation is: $\text{Cr}_2\text{O}_7^{2-}(aq) + 14\text{H}^+(aq) + 6\text{Fe}^{2+}(aq) \rightarrow 2\text{Cr}^{3+}(aq) + 6\text{Fe}^{3+}(aq) + 7\text{H}_2\text{O}(l)$. A $5.00\,\text{g}$ portion of spathose was treated with an excess of concentrated hydrochloric acid and then filtered. The filtrate was then diluted to $250\,\text{cm}^3$ in a volumetric flask with distilled water. A $25.0\,\text{cm}^3$ portion of this solution needed $27.30\,\text{cm}^3$ of $0.0200\,\text{mol dm}^{-3}$ dichromate(VI) solution for the reaction to go to completion.
(a(i))[1]
Calculate the amount, in moles, of dichromate(VI) ions used in the titration.
(a(ii))[1]
Use your answer to (i) to calculate the amount, in moles, of $\text{Fe}^{2+}$ present in the $25.0\,\text{cm}^3$ sample.
(a(iii))[1]
Use your answer to (ii) to calculate the amount, in moles, of $\text{Fe}^{2+}$ present in the $250\,\text{cm}^3$ volumetric flask.
(a(iv))[2]
Use your answer to (iii) to calculate the mass of iron(II) carbonate present in the sample of spathose.
(a(v))[1]
Calculate the percentage of iron(II) carbonate in the sample of spathose.
(b(i))[2]
Write an ionic equation for this reaction. Do not include state symbols.
(b(ii))[2]
Any surplus tin(II) chloride may be removed by allowing it to react with $\text{HgCl}_2(aq)$. A white precipitate of $\text{Hg}_2\text{Cl}_2$ forms. Complete the equation for this reaction.
$\ldots\ldots(\ldots) + \ldots\text{HgCl}_2(aq) \rightarrow \text{SnCl}_4(\ldots) + \text{Hg}_2\text{Cl}_2(\ldots)$
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “$0.0200 \times \frac{27.30}{1000} = 5.46 \times 10^{-4}\,\text{mol}$” …