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9701/33

Chemistry 9701/33 — October/November 2023

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

3
questions
40
marks
120
minutes

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

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

You will carry out a titration to determine the concentration of a solution of potassium manganate(VII), KMnO4\text{KMnO}_4. Potassium manganate(VII) reacts with excess acidified potassium iodide to produce iodine. This iodine is titrated with aqueous sodium thiosulfate.

FA 1 is aqueous potassium manganate(VII), KMnO4\text{KMnO}_4.
FA 2 is 0.500 mol dm−30.500 \text{ mol dm}^{-3} potassium iodide, KI\text{KI}.
FA 3 is 0.120 mol dm−30.120 \text{ mol dm}^{-3} sodium thiosulfate, Na2S2O3\text{Na}_2\text{S}_2\text{O}_3.
FA 4 is dilute sulfuric acid, H2SO4\text{H}_2\text{SO}_4.
FA 5 is starch indicator.

(a)

Method

Titration

  • Fill the burette with FA 3.
  • Pipette 25.0 cm325.0 \text{ cm}^3 of FA 1 into a conical flask.
  • Use the 25 cm325 \text{ cm}^3 measuring cylinder to add 10.0 cm310.0 \text{ cm}^3 of FA 2 to the conical flask.
  • Use the 50 cm350 \text{ cm}^3 measuring cylinder to add 15.0 cm315.0 \text{ cm}^3 of FA 4 to the conical flask.
  • Perform a rough titration by adding FA 3 from the burette to the conical flask until the solution is yellow.
  • Then add several drops of FA 5. Continue the titration until the mixture in the flask becomes colourless. This is the end-point.
  • Record the initial and final burette readings in the space below.

The rough titre is ....................................................... cm3\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 your burette readings and the volume of FA 3 added in each accurate titration.

Keep FA 2 and FA 5 for use in Question 3(b).

7M
(b)

From your accurate titration results, calculate a suitable mean value to be used in your calculations. Show clearly how you obtained the mean value.

The iodine produced by FA 1 required .................................... cm3\text{cm}^3 of FA 3.

1M
(c)
(i)

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

1M
(ii)

Calculate the amount, in mol, of sodium thiosulfate in the volume of FA 3 calculated in (b).

amount of Na2S2O3\text{Na}_2\text{S}_2\text{O}_3 = .................................................. mol

1M
(iii)

The reaction by which iodine is produced is shown.

2KMnO4(aq)+10KI(aq)+8H2SO4(aq)→6K2SO4(aq)+2MnSO4(aq)+5I2(aq)+8H2O(l)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.

2Na2S2O3(aq)+I2(aq)→2NaI(aq)+Na2S4O6(aq)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})

Calculate the concentration of potassium manganate(VII), in mol dm−3\text{mol dm}^{-3}, in FA 1.

concentration of KMnO4\text{KMnO}_4 = .......................................... mol dm−3\text{mol dm}^{-3}

2M
(iv)

A laboratory technician purchased a bottle containing 50.00 g50.00 \text{ g} of potassium manganate(VII), KMnO4\text{KMnO}_4, for this practical examination. Using your answer to (c)(iii), calculate the maximum volume of FA 1, in dm3\text{dm}^3, that the technician can prepare using the contents of this bottle.

volume of FA 1 = .................................................. dm3\text{dm}^3

1M
(d)

A student suggested that the accuracy of the experiment would be increased by using a 10 cm310 \text{ cm}^3 pipette to measure FA 2.

State whether the student is correct. Explain your answer.

1M

The rest of this paper

2 more questions
  • Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
  • Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation14M
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