Chemistry 9701/53 — October/November 2020

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Analysis, Conclusions and Evaluation · Planning

Q1Analysis, Conclusions and EvaluationPlanningFree sample

Aqueous potassium manganate(VII) can be used to determine the amount of iron present in a sample of iron wire by redox titration. Before potassium manganate(VII) can be used, its concentration must be determined using aqueous sodium ethanedioate made from the hydrated solid $\text{Na}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}$ .

(a)

(i)

Calculate the mass of $\text{Na}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}$ required to make $250.0\text{ cm}^3$ of $0.200\text{ mol dm}^{-3}$ sodium ethanedioate standard solution.

$[A_r: \text{Na}, 23.0; \text{C}, 12.0; \text{O}, 16.0; \text{H}, 1.0]$

(ii)

Describe how the student should accurately prepare $250.0\text{ cm}^3$ of $0.200\text{ mol dm}^{-3}$ sodium ethanedioate standard solution from the weighed sample of $\text{Na}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}$ of mass calculated in (a)(i).

In your description you should include the names and capacities of any apparatus used.

(b)

Ethanedioate ions, $\text{C}_2\text{O}_4^{2-}(\text{aq})$ , react with manganate(VII) ions, $\text{MnO}_4^-(\text{aq})$ , according to the ionic equation shown.

2\text{MnO}_4^-(\text{aq}) + 16\text{H}^+(\text{aq}) + 5\text{C}_2\text{O}_4^{2-}(\text{aq}) \rightarrow 2\text{Mn}^{2+}(\text{aq}) + 8\text{H}_2\text{O}(\text{l}) + 10\text{CO}_2(\text{g})

$25.0\text{ cm}^3$ of $0.200\text{ mol dm}^{-3}$ $\text{C}_2\text{O}_4^{2-}(\text{aq})$ required $18.40\text{ cm}^3$ $\text{MnO}_4^-(\text{aq})$ for complete reaction.

Calculate the concentration of the aqueous potassium manganate(VII). Give your answer to three significant figures.

(c)

In another experiment, a student uses $0.0200\text{ mol dm}^{-3}$ $\text{MnO}_4^-(\text{aq})$ to analyse the percentage of iron in a sample of iron wire using the following method.

step 1 The mass of the iron wire is recorded.

step 2 The iron wire is dissolved in $20\text{ cm}^3$ , an excess, of sulfuric acid and made up to a volume of $250.0\text{ cm}^3$ with distilled water. The iron reacts and dissolves in sulfuric acid to form $\text{Fe}^{2+}(\text{aq})$ ions.

step 3 A $25.0\text{ cm}^3$ sample of this $\text{Fe}^{2+}$ containing solution is titrated with $0.0200\text{ mol dm}^{-3}$ $\text{MnO}_4^-(\text{aq})$ .

The ionic equation for the reaction between $\text{MnO}_4^-(\text{aq})$ and $\text{Fe}^{2+}(\text{aq})$ is shown.

\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{Fe}^{2+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 5\text{Fe}^{3+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})

The student's results are shown in the table.

$[A_r: \text{Fe}, 55.8]$

	rough	titration 1	titration 2	titration 3	titration 4
final burette reading/ $\text{cm}^3$	45.50	44.75	44.45	44.80	44.40
initial burette reading/ $\text{cm}^3$	0.00	0.10	0.15	0.00	0.00
titre/ $\text{cm}^3$	45.50	44.65	44.30	44.80	44.40

(i)

Circle the titres the student should use to obtain the most accurate value for the volume of $0.0200\text{ mol dm}^{-3}$ $\text{KMnO}_4$ that is needed to react with $25.0\text{ cm}^3$ of the prepared iron solution. Explain your answer.

(ii)

The burette used for the titration has graduations every $0.10\text{ cm}^3$ .

Calculate the maximum percentage error in the titre of titration 2.

Show your working.

(iii)

Derive an expression to show how you would calculate the percentage by mass of iron in the iron wire.

Use $x$ to represent the average titre and $y$ to represent the mass of iron wire used.

(iv)

The student left the solution of $\text{Fe}^{2+}(\text{aq})$ in sulfuric acid without a stopper for a few days. The student repeated the titration and found that the average titre was lower.

Suggest why.

(v)

In step 2, the sulfuric acid is used to dissolve the iron in the iron wire.

Suggest the other function of the sulfuric acid in this experiment.

(d)

Name an appropriate piece of apparatus to measure the volume of sulfuric acid in step 2. Give a reason for your answer.

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