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9701/31

Chemistry 9701/31 — October/November 2022

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

Group 1 elements form salts with ethanedioic acid. These salts are ethanedioates and have the formula (COOM)2⋅2H2O(\text{COOM})_2 \cdot 2\text{H}_2\text{O}, where M\text{M} is the Group 1 metal.

Ethanedioate ions react with manganate(VII) ions as shown.

5(COO−)2(aq)+2MnO4−(aq)+16H+(aq)→10CO2(g)+2Mn2+(aq)+8H2O(l)5(\text{COO}^-)_2(\text{aq}) + 2\text{MnO}_4^-(\text{aq}) + 16\text{H}^+(\text{aq}) \rightarrow 10\text{CO}_2(\text{g}) + 2\text{Mn}^{2+}(\text{aq}) + 8\text{H}_2\text{O}(\text{l})

You will determine which metal is present in (COOM)2⋅2H2O(\text{COOM})_2 \cdot 2\text{H}_2\text{O} by titrating a solution of this salt with manganate(VII) ions.

  • FA 1 is 10.14 g dm−310.14\text{ g dm}^{-3} aqueous hydrated ethanedioate of metal M\text{M}, (COOM)2⋅2H2O(\text{COOM})_2 \cdot 2\text{H}_2\text{O}.
  • FA 2 is 0.0200 mol dm−30.0200\text{ mol dm}^{-3} potassium manganate(VII), KMnO4\text{KMnO}_4.
  • FA 3 is 1 mol dm−31\text{ mol dm}^{-3} sulfuric acid, H2SO4\text{H}_2\text{SO}_4.
(a)

Method

  • Fill the burette with FA 2.
  • Pipette 25.0 cm325.0\text{ cm}^3 of FA 1 into a conical flask.
  • Use the measuring cylinder to add approximately 20 cm320\text{ cm}^3 of FA 3 into the conical flask.
  • Place the conical flask on a tripod and gauze and heat carefully until the temperature of the solution is approximately 70 ∘C70\text{ }^\circ\text{C}.
  • Remove the flame.
  • Carefully lift the hot conical flask and place it on the white tile under the burette.
  • During titrations, add FA 2, slowly at first, until a permanent pale pink colour is formed. The pink colour on initial addition may take several seconds to disappear.
  • If the reaction mixture turns brown, reheat it to about 70 ∘C70\text{ }^\circ\text{C}. If the brown colour disappears, continue with the titration. If the brown colour remains, discard the contents of the flask and begin a new titration.
  • Perform a rough titration with FA 2. 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 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 2 added in each accurate titration.
7M
(b)

From your accurate titration results, calculate a suitable mean value to be used in your calculations.

Show clearly how you obtained this value.

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

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 manganate(VII) ions, MnO4−\text{MnO}_4^-, in the volume of FA 2 calculated in (b).

amount of MnO4−=.............................. mol\text{amount of MnO}_4^- = \text{.............................. mol}
1M
(iii)

Calculate the amount, in mol, of ethanedioate ions that reacted with the manganate(VII) ions in (c)(ii).

amount of (COO−)2=.............................. mol\text{amount of (COO}^-)_2 = \text{.............................. mol}

Hence calculate the concentration, in mol dm−3\text{mol dm}^{-3}, of ethanedioate ions in FA 1.

concentration of (COO−)2=.............................. mol dm−3\text{concentration of (COO}^-)_2 = \text{.............................. mol dm}^{-3}
1M
(iv)

Calculate the relative formula mass, MrM_r, of the hydrated ethanedioate, (COOM)2⋅2H2O(\text{COOM})_2 \cdot 2\text{H}_2\text{O}.

Mr=..............................M_r = \text{..............................}
1M
(v)

Identify M. Show your working.

M is .............................. .

2M
(d)

Explain why it is necessary to add FA 3 in each titration.

1M

The rest of this paper

2 more questions
  • Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation10M
  • Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation15M
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