Chemistry 9701/33 — October/November 2016

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

questions

marks

120

minutes

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

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

Hydrogen peroxide, $\text{H}_2\text{O}_2$ , can be oxidised to give oxygen, $\text{O}_2$ . This reaction happens rapidly in the presence of acidified potassium manganate(VII), $\text{KMnO}_4$ .

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

You will determine the concentration of a solution of hydrogen peroxide. You will first dilute the solution and then carry out a titration using acidified potassium manganate(VII), $\text{KMnO}_4$ .

FA 1 is aqueous hydrogen peroxide, $\text{H}_2\text{O}_2$ .
FA 2 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 3 is $1.0\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Dilution

Pipette $25.0\text{ cm}^3$ of FA 1 into the $250\text{ cm}^3$ volumetric flask.
Add distilled water to make $250\text{ cm}^3$ of solution and shake the flask thoroughly.
Label this diluted solution of hydrogen peroxide FA 4.

Titration

Fill the burette with FA 2.
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 measuring cylinder to add $25\text{ cm}^3$ of FA 3 to the 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 certain any recorded results show the precision of your practical work.
Record in a suitable form below all of your burette readings and the volume of FA 2 added in each accurate titration.

Keep FA 3 and FA 4 for use in Question 2.

(b)

From your accurate titration results, obtain a suitable value for the volume of FA 2 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 2.

(c)

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 2 calculated in (b).

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

(ii)

Use your answer to (i) and the equation at the top of page 2 to calculate the number of moles of hydrogen peroxide present in $25.0\text{ cm}^3$ of FA 4.

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

(iii)

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

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

(iv)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of $\text{H}_2\text{O}_2$ in FA 1.

concentration of $\text{H}_2\text{O}_2$ in FA 1 = ............................. $\text{mol dm}^{-3}$

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis · Presentation of Data and Observations17M

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