Chemistry 9701/34 — May/June 2021

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

questions

marks

120

minutes

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

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

Quantitative analysis

Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided.

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

You will carry out a titration to determine the concentration of a solution of potassium manganate(VII). You will react potassium manganate(VII) with excess acidified potassium iodide to produce iodine. You will then titrate the iodine with sodium thiosulfate.

FB 1 is hydrated sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3 \cdot 5\text{H}_2\text{O}$ .
FB 3 is aqueous potassium manganate(VII), $\text{KMnO}_4$ .
FB 4 is $0.50\text{ mol dm}^{-3}$ potassium iodide, $\text{KI}$ .
FB 5 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .
starch indicator

(a)

Method

Preparing a solution of FB 1

Weigh the stoppered container of FB 1. Record the mass in the space below.
Tip all the FB 1 into the beaker.
Reweigh the container with its stopper. Record the mass.
Calculate and record the mass of FB 1 used.
Add approximately $100\text{ cm}^3$ of distilled water to the FB 1 in the beaker.
Stir the mixture with a glass rod until all the FB 1 has dissolved.
Transfer this solution into the $250\text{ cm}^3$ volumetric flask.
Wash the beaker with distilled water and transfer the washings to the volumetric flask.
Rinse the glass rod with distilled water and transfer the washings to the volumetric flask.
Make the solution in the volumetric flask up to the mark using distilled water.
Shake the flask thoroughly.
This solution of sodium thiosulfate is FB 2. Label the flask FB 2.

Titration

Fill the burette with FB 2.
Pipette $25.0\text{ cm}^3$ of FB 3 into a conical flask.
Use the $25\text{ cm}^3$ measuring cylinder to add $15\text{ cm}^3$ of FB 5 to the conical flask.
Use the same measuring cylinder to add $10\text{ cm}^3$ of FB 4 to the conical flask.
Perform a rough titration by adding FB 2 from the burette to the conical flask until the solution is yellow. Then add several drops of starch indicator and continue the titration until the mixture in the flask becomes colourless. This is the end-point.

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 in a suitable form below all of your burette readings and the volume of FB 2 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations.
Show clearly how you obtained this value.

The iodine produced by FB 3 required .............................. $\text{cm}^3$ of FB 2.

(c)

Calculations

(i)

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

(ii)

Calculate the number of moles of hydrated sodium thiosulfate, FB 1, that you weighed.

moles of $\text{Na}_2\text{S}_2\text{O}_3 \cdot 5\text{H}_2\text{O}$ = .............................. mol

(iii)

Calculate the number of moles of sodium thiosulfate in the volume of FB 2 calculated in (b).

moles of $\text{Na}_2\text{S}_2\text{O}_3$ = .............................. mol

(iv)

The reaction by which iodine is produced is shown.

2\text{KMnO}_4(\text{aq}) + 10\text{KI}(\text{aq}) + 8\text{H}_2\text{SO}_4(\text{aq}) \rightarrow 6\text{K}_2\text{SO}_4(\text{aq}) + 2\text{MnSO}_4(\text{aq}) + 5\text{I}_2(\text{aq}) + 8\text{H}_2\text{O}(\text{l})

During the titration, sodium thiosulfate reacts with the iodine produced.

2\text{Na}_2\text{S}_2\text{O}_3(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow 2\text{NaI}(\text{aq}) + \text{Na}_2\text{S}_4\text{O}_6(\text{aq})

Use your answer to (c)(iii) to calculate the concentration of $\text{KMnO}_4$ , in $\text{mol dm}^{-3}$ , in FB 3.

concentration of $\text{KMnO}_4$ = .............................. $\text{mol dm}^{-3}$

(v)

Calculate the mass of $\text{KMnO}_4$ needed to prepare $1.00\text{ dm}^3$ of FB 3. Show your working.

mass of $\text{KMnO}_4$ = .............................. g

(d)

(i)

Solution FB 3 was actually prepared by dissolving $3.16\text{ g}$ of $\text{KMnO}_4$ in $1.00\text{ dm}^3$ of solution.

Show how you would use your answer to (c)(v) to calculate the overall percentage error in your experiment.

(ii)

A student suggested that the percentage error in the experiment would be reduced by using a $10\text{ cm}^3$ pipette to measure FB 4.

State whether the student is correct. Explain your answer.

The rest of this paper

2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation13M

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