Chemistry 9701/36 — October/November 2012

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

You are to determine the concentration of aqueous copper(II) sulfate by titration. The concentration of $\text{Cu}^{2+}$ ions in a solution can be found by reaction with an excess of aqueous iodide ions to produce iodine. The amount of iodine formed can be found by titration with thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ .

FB 1 is aqueous copper(II) sulfate, $\text{CuSO}_4$ .
FB 2 is $0.100\text{ mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
FB 3 is aqueous potassium iodide, $\text{KI}$ .
starch indicator

Read through the instructions carefully before starting any practical work.

(a)

Method

Fill the burette with FB 2.
Pipette $25.0\text{ cm}^3$ of FB 1 into a conical flask.
Use a measuring cylinder to add $10\text{ cm}^3$ of FB 3 into the conical flask.
Titrate this mixture with FB 2 until the colour of the mixture changes from brown to yellow-brown. An off-white precipitate will also be present in the flask throughout the titration.
Add approximately $1\text{ cm}^3$ of starch indicator.
Continue the titration until the blue-black colour of the starch-iodine complex just disappears leaving the off-white precipitate.
Perform a rough titration and record your burette readings in the space below.

The rough titre is ................................ $\text{cm}^3$ .

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 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 to be used in your calculations. Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FB 1 required ........................ $\text{cm}^3$ of FB 2.

(c)

The equations for the formation of iodine and its reaction with thiosulfate ions are given below.

2\text{Cu}^{2+} + 4\text{I}^- \rightarrow 2\text{CuI} + \text{I}_2

\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow \text{S}_4\text{O}_6^{2-} + 2\text{I}^-

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

(i)

Calculate the number of moles of thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ , present in the volume of FB 2 in (b).

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

(ii)

Using the equations above, deduce the number of moles of $\text{Cu}^{2+}$ ions present in each $25.0\text{ cm}^3$ .

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

(iii)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of copper(II) sulfate in FB 1.

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

(d)

Three students repeated the experiment but each obtained different values for the concentration of $\text{CuSO}_4$ .

The students each suggested possible improvements.

Student 1 suggested that a larger quantity of starch should be added.
Student 2 suggested that a larger volume of potassium iodide, FB 3, should be added.
Student 3 suggested that the contents of the conical flask should be filtered before titration.

Comment on the effectiveness of each of these possible improvements. Explain your answers.

Student 1

...........................................................................................................................................

Student 2

...........................................................................................................................................

Student 3

...........................................................................................................................................

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation14M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation14M

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