HardChemistryHigh School (11–12)The p-Block ElementsNumericalConceptual

A 25.0 mL sample of a solution of sulphur dioxide needs 20.0 mL of 0.0200 mol/L acidified potassium…

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A $25.0\ \text{mL}$ sample of a solution of sulphur dioxide needs $20.0\ \text{mL}$ of $0.0200\ \text{mol/L}$ acidified potassium permanganate to reach the end point. The equation is $5\text{SO}_2 + 2\text{MnO}_4^- + 2\text{H}_2\text{O} \rightarrow 5\text{SO}_4^{2-} + 2\text{Mn}^{2+} + 4\text{H}^+$ . Find the amount of sulphur dioxide in the sample, its concentration in mol/L, and its mass concentration in g/L, with $64.0\ \text{g/mol}$ . A second $25.0\ \text{mL}$ sample is titrated with $0.0100\ \text{mol/L}$ acidified potassium dichromate, by $\text{Cr}_2\text{O}_7^{2-} + 3\text{SO}_2 + 2\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 3\text{SO}_4^{2-} + \text{H}_2\text{O}$ . Find the volume of dichromate.

Need a reminder? The key ideaShow

Ozone gives one mole of iodine for each mole of ozone with potassium iodide. Roasting pyrites follows 4FeS2 + 11O2 giving 2Fe2O3 + 8SO2, so each FeS2 gives 2 SO2. Permanganate and dichromate oxidise sulphur dioxide: 5 SO2 for each 2 $\text{MnO}_{4}^{-}$ and 3 SO2 for each $\text{Cr}_{2}\text{O}_{7}^{2-}$ . Work in moles: the amount of a solute is the volume in litres times the concentration.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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