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9701/33

Chemistry 9701/33 — October/November 2015

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

The formula of hydrated copper(II) sulfate is CuSO4⋅xH2O\text{CuSO}_4 \cdot x\text{H}_2\text{O} where xx is the number of moles of water of crystallisation in one mole of salt. You will determine the value of xx by titration.

When aqueous copper(II) ions react with aqueous iodide ions, I−\text{I}^-, iodine is produced.

2Cu2+(aq)+4I−(aq)→I2(aq)+2CuI(s)2\text{Cu}^{2+}(\text{aq}) + 4\text{I}^-(\text{aq}) \rightarrow \text{I}_2(\text{aq}) + 2\text{CuI}(\text{s})

The amount of iodine, I2\text{I}_2, produced can be found by titrating it with aqueous thiosulfate ions, S2O32−\text{S}_2\text{O}_3^{2-}.

2S2O32−(aq)+I2(aq)→S4O62−(aq)+2I−(aq)2\text{S}_2\text{O}_3^{2-}(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow \text{S}_4\text{O}_6^{2-}(\text{aq}) + 2\text{I}^-(\text{aq})

FA 1 is aqueous CuSO4⋅xH2O\text{CuSO}_4 \cdot x\text{H}_2\text{O} containing 26.2 g dm−326.2\text{ g dm}^{-3}.
FA 2 is 0.100 mol dm−30.100\text{ mol dm}^{-3} sodium thiosulfate, Na2S2O3\text{Na}_2\text{S}_2\text{O}_3.
FA 3 is aqueous potassium iodide, KI\text{KI}.
starch indicator

(a)

Method

  • Pipette 25.0 cm325.0\text{ cm}^3 of FA 1 into a conical flask.
  • Use the measuring cylinder to add 15 cm315\text{ cm}^3 of FA 3, an excess of KI\text{KI}, to the conical flask. The solution will turn brown because iodine is formed.
  • Fill the burette with FA 2.
  • Add FA 2 from the burette until the colour of the mixture changes to pale brown.
  • Add 10 drops of starch indicator. The mixture will turn blue-black.
  • Continue adding FA 2 from the burette until the dark colour suddenly disappears to leave an off-white solid. This is the end point of the titration.
  • Carry out a rough titration and record your 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 certain 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 2 added in each accurate titration.
7M
(b)

From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

25.0 cm325.0\text{ cm}^3 of FA 1 required ............................. cm3\text{cm}^3 of FA 2.

1M
(c)

Calculations

Show your working and appropriate significant figures in each step of your calculations.

6M
(i)

Calculate the number of moles of thiosulfate ions present in the volume of FA 2 you have calculated in (b).

moles of S2O32−\text{S}_2\text{O}_3^{2-} = ............................. mol

(ii)

Use your answer to (i), and the equations for the reactions involved, to deduce the number of moles of Cu2+\text{Cu}^{2+} present in 25.0 cm325.0\text{ cm}^3 of FA 1.

2Cu2+(aq)+4I−(aq)→I2(aq)+2CuI(s)2\text{Cu}^{2+}(\text{aq}) + 4\text{I}^-(\text{aq}) \rightarrow \text{I}_2(\text{aq}) + 2\text{CuI}(\text{s}) 2S2O32−(aq)+I2(aq)→S4O62−(aq)+2I−(aq)2\text{S}_2\text{O}_3^{2-}(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow \text{S}_4\text{O}_6^{2-}(\text{aq}) + 2\text{I}^-(\text{aq})

moles of Cu2+\text{Cu}^{2+} = ............................. mol

(iii)

Use your answer to (ii) and the mass of CuSO4⋅xH2O\text{CuSO}_4 \cdot x\text{H}_2\text{O} present in the solution, to calculate the relative molecular mass, MrM_r, of CuSO4⋅xH2O\text{CuSO}_4 \cdot x\text{H}_2\text{O}.

MrM_r of CuSO4⋅xH2O\text{CuSO}_4 \cdot x\text{H}_2\text{O} = .............................

(iv)

Determine the value of xx.
(Use data from the Periodic Table on page 12.)

xx = .............................

The rest of this paper

2 more questions
  • Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
  • Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation15M
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