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9701/31

Chemistry 9701/31 — May/June 2017

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 · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Qualitative Analysis

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

In this experiment you will determine the relative formula mass of a copper salt by titration.

A solution of the copper salt reacts with excess acidified potassium iodide, producing iodine. This iodine is then titrated with aqueous sodium thiosulfate, using starch indicator.

FA 1 is an aqueous solution of the copper salt prepared by dissolving 26.0 g26.0\text{ g} of the salt to make 1.00 dm31.00\text{ dm}^3 of solution.
FA 2 is dilute sulfuric acid, H2SO4\text{H}_2\text{SO}_4.
FA 3 is aqueous potassium iodide, KI\text{KI}.
FA 4 is 0.110 mol dm−30.110\text{ mol dm}^{-3} sodium thiosulfate, Na2S2O3\text{Na}_2\text{S}_2\text{O}_3.
starch indicator

(a)

Method

  • Fill the burette with FA 4.
  • Pipette 25.0 cm325.0\text{ cm}^3 of FA 1 into a conical flask.
  • Use the measuring cylinder to add approximately 10 cm310\text{ cm}^3 of FA 2 to the same conical flask.
  • Use the measuring cylinder to add approximately 20 cm320\text{ cm}^3 of FA 3 to the mixture in the conical flask. The mixture will now be a brown colour, due to iodine produced in the reaction.
  • Begin your rough titration by adding FA 4 from the burette until the mixture becomes light brown.
  • Add 10 drops of starch indicator. The mixture will become darker.
  • Continue titrating until the mixture becomes an off-white colour. This is the end-point.
  • Add one drop of starch indicator to check that no traces of dark colour are produced. If the mixture stays off-white, the titration is finished. If some dark colour is produced, because iodine is still present, continue the titration.
  • Record your burette readings and the rough titre 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 of your burette readings and the volume of FA 4 added in each accurate titration.

Keep FA 3 and starch indicator for use in Question 3.

7M
(b)

From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations.
Show clearly how you obtained this value.

The iodine produced required ............................. cm3\text{cm}^3 of FA 4.

1M
(c)

Calculations

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

4M
(i)

Calculate the number of moles of sodium thiosulfate, Na2S2O3\text{Na}_2\text{S}_2\text{O}_3, in the volume of FA 4 calculated in (b).

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

(ii)

Balance the equation for the reaction of iodine with sodium thiosulfate. State symbols are not required.

......I2+........Na2S2O3→........Na2S4O6+........NaI\text{......I}_2 + \text{........Na}_2\text{S}_2\text{O}_3 \rightarrow \text{........Na}_2\text{S}_4\text{O}_6 + \text{........NaI}
(iii)

Using your answer to (ii), calculate the number of moles of iodine that reacted with the number of moles of Na2S2O3\text{Na}_2\text{S}_2\text{O}_3 calculated in (i).

moles of I2\text{I}_2 = ............................. mol

(iv)

Iodine, I2\text{I}_2, is produced in the reaction between FA 1 and FA 3. FA 3 is in excess.

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

Using your answer to (iii), calculate the number of moles of copper(II) ions in 25.0 cm325.0\text{ cm}^3 of FA 1.

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

(v)

Using your answer to (iv) and the information on page 2, calculate the relative formula mass of the copper compound in FA 1.

MrM_r of copper compound = .............................

The rest of this paper

2 more questions
  • Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation14M
  • Q3Qualitative Analysis14M
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