Chemistry 9701/33 — May/June 2014

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

You are to determine, by titration, the change in oxidation number of a transition metal ion, $\text{M}^{2+}$ , when reacted with acidified potassium manganate(VII).

FA 1 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 2 is $0.0530\text{ mol dm}^{-3}$ transition metal salt, $\text{MSO}_4$ .
FA 3 is $1.0\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Fill the burette with FA 1.
Pipette $25.0\text{ cm}^3$ of FA 2 into the conical flask.
Use the measuring cylinder to add $25\text{ cm}^3$ of FA 3 into the conical flask.
Carry out a rough titration and record your burette readings in the space below. Add FA 1 until the contents of the flask turn a permanent pale pink colour.

The rough titre is ......................... $\text{cm}^3$ .

Carry out as many accurate titrations as you think necessary to obtain consistent results.
Make certain any recorded results show the precision of your practical work.
Record below, in a suitable form, all of your burette readings and the volume of FA 1 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

$25.0\text{ cm}^3$ of FA 2 required .......................... $\text{cm}^3$ of FA 1.

(c)

Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i)

Calculate the number of moles of potassium manganate(VII) present in the volume of FA 1 calculated in (b).

$\text{moles of KMnO}_4 = \text{.......................... mol}$

(ii)

Calculate the number of moles of $\text{MSO}_4$ in $25.0\text{ cm}^3$ of FA 2.

$\text{moles of MSO}_4\text{ in }25.0\text{ cm}^3 = \text{.......................... mol}$

(iii)

Use your answers to (i) and (ii) to calculate the number of moles of $\text{MSO}_4$ that react with $1\text{ mole}$ of $\text{KMnO}_4$ .

$\text{moles of MSO}_4 = \text{.......................... mol}$

(iv)

Two possible equations for the reaction of acidified $\text{KMnO}_4$ with $\text{MSO}_4$ are below.

\begin{aligned} \text{equation 1} \quad &2\text{KMnO}_4 + 10\text{MSO}_4 + 8\text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 5\text{M}_2(\text{SO}_4)_3 + 8\text{H}_2\text{O} \\ \text{equation 2} \quad &2\text{KMnO}_4 + 5\text{MSO}_4 + 8\text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 5\text{M}(\text{SO}_4)_2 + 8\text{H}_2\text{O} \end{aligned}

State and explain which of these two equations is consistent with your answer to (iii).

(v)

Use your answer to (iv) to state the oxidation number of the transition metal $\text{M}$ in the product of the reaction.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis · Presentation of Data and Observations15M

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