Chemistry 9701/31 — 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

Hydrogen peroxide, $\text{H}_2\text{O}_2$ , is unstable and decomposes to give water and oxygen. In addition to the usual units of concentration, $\text{mol dm}^{-3}$ , the concentration of a solution of hydrogen peroxide can also be given in ‘volume strength’ or ‘vol’. For example, in Question 3 you will use a solution of ‘20 vol’ $\text{H}_2\text{O}_2$ . This term means that when $1\text{ dm}^3$ of this solution is completely decomposed it generates $20\text{ dm}^3$ of oxygen at room temperature and pressure.

The aim of the following titration is to determine the volume strength of a solution of hydrogen peroxide. To do this you will titrate an acidified solution of hydrogen peroxide with potassium manganate(VII) solution.

FA 1 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 2 is aqueous hydrogen peroxide, $\text{H}_2\text{O}_2$ .
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 to the conical flask.
Run FA 1 from the burette into the conical flask until the pink colour remains.
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 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 obtained this value.

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

(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 1 calculated in (b).

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

(ii)

Use the following equation and your answer to (i) to calculate the number of moles of hydrogen peroxide used in each titration.

2\text{KMnO}_4 + 5\text{H}_2\text{O}_2 + 3\text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 5\text{O}_2 + 8\text{H}_2\text{O}

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 2.

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

(iv)

Write an equation for the decomposition of hydrogen peroxide to produce oxygen and water.

(v)

Calculate the concentration in ‘volume strength’ of $\text{H}_2\text{O}_2$ in FA 2. You must assume that the molar volume of oxygen is $24.0\text{ dm}^3\text{ mol}^{-1}$ at room temperature and pressure.

concentration of FA 2 = ........................ ‘vol’

The rest of this paper

2 more questions

Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation13M
Q3Qualitative Analysis9M

Loading the full paper…