Chemistry 9701/33 — February/March 2020

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

The concentrations of solutions of hydrogen peroxide are often represented in terms of ‘volume strength’. In this experiment you will determine the ‘volume strength’ of a solution of hydrogen peroxide by titration with acidified potassium manganate(VII).

2\text{MnO}_4^-(\text{aq}) + 5\text{H}_2\text{O}_2(\text{aq}) + 6\text{H}^+(\text{aq}) \rightarrow 2\text{Mn}^{2+}(\text{aq}) + 8\text{H}_2\text{O}(\text{l}) + 5\text{O}_2(\text{g})

FA 1 is $0.0300\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 2 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .
FA 3 is aqueous hydrogen peroxide, $\text{H}_2\text{O}_2$ .

(a)

Method

Dilution of FA 3

Pipette $25.0\text{ cm}^3$ of FA 3 into the $250\text{ cm}^3$ volumetric flask.
Make the solution up to the mark using distilled water.
Shake the flask thoroughly.
Label this diluted solution of hydrogen peroxide FA 4.

Titration

Fill the burette with FA 1.
Rinse the pipette thoroughly with distilled water and then with a little FA 4.
Pipette $25.0\text{ cm}^3$ of FA 4 into a conical flask.
Use the $25\text{ cm}^3$ measuring cylinder to add $20\text{ cm}^3$ of FA 2 into the same conical flask.
Perform a rough titration and record your burette readings in the space below.

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

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

Keep FA 1, FA 2 and FA 3 for use in Questions 2 and 3.

(b)

From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations.
Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FA 4 required .............................. $\text{cm}^3$ of FA 1.

(c)

Calculations

(i)

Give your answers to (ii), (iii), (iv) and (v) to the appropriate number of significant figures.

(ii)

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

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

(iii)

The equation for the reaction of potassium manganate(VII) with hydrogen peroxide is shown.

2\text{MnO}_4^-(\text{aq}) + 5\text{H}_2\text{O}_2(\text{aq}) + 6\text{H}^+(\text{aq}) \rightarrow 2\text{Mn}^{2+}(\text{aq}) + 8\text{H}_2\text{O}(\text{l}) + 5\text{O}_2(\text{g})

Use your answer to (c)(ii) to calculate the number of moles of hydrogen peroxide used in each titration.

\text{moles of H}_2\text{O}_2 = \text{.............................. mol}

Hence calculate the concentration of $\text{H}_2\text{O}_2$ in FA 4, in $\text{mol dm}^{-3}$ .

\text{concentration of H}_2\text{O}_2\text{ in FA 4} = \text{.............................. mol dm}^{-3}

(iv)

Calculate the concentration of $\text{H}_2\text{O}_2$ in FA 3, in $\text{mol dm}^{-3}$ .

\text{concentration of H}_2\text{O}_2\text{ in FA 3} = \text{.............................. mol dm}^{-3}

(v)

When hydrogen peroxide decomposes in the presence of a catalyst, oxygen is produced.

\text{H}_2\text{O}_2(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l}) + \frac{1}{2}\text{O}_2(\text{g})

The ‘volume strength’ of hydrogen peroxide is equal to the volume of oxygen, in $\text{dm}^3$ , produced under room conditions, when $1.00\text{ dm}^3$ of the solution decomposes.

Use your answer to (c)(iv) and the equation above to calculate the volume, in $\text{dm}^3$ , of oxygen produced when $1.00\text{ dm}^3$ of FA 3 decomposes. This is the ‘volume strength’, in vol, of FA 3.

(Under room conditions $1.00\text{ mol}$ of gas occupies a volume of $24.0\text{ dm}^3$ .
If you were unable to calculate the concentration of $\text{H}_2\text{O}_2$ in FA 3, assume that it is $1.02\text{ mol dm}^{-3}$ . This may not be the correct value.)

\text{‘volume strength’ of FA 3} = \text{.............................. vol}

(d)

The maximum error in reading a $25.0\text{ cm}^3$ pipette is $\pm 0.06\text{ cm}^3$ .

Show by calculation that the pipette is more accurate than a burette for measuring $25.0\text{ cm}^3$ of solution.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis14M

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