Chemistry 9701/33 — October/November 2017

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

questions

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 value of $x$ in the formula for hydrated copper(II) sulfate, $\text{CuSO}_4 \cdot x\text{H}_2\text{O}$ . You will first react a solution of $\text{Cu}^{2+}$ ions with excess iodide ions, $\text{I}^-$ . This reaction produces iodine.

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

The amount of iodine produced can be determined by titrating with thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ .

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

FA 1 is $0.150\text{ mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
FA 2 is dilute sulfuric acid.
FA 3 is $1.00\text{ mol dm}^{-3}$ potassium iodide, $\text{KI}$ .
FA 4 is a solution made by dissolving $32.5\text{ g}$ of $\text{CuSO}_4 \cdot x\text{H}_2\text{O}$ in $1.00\text{ dm}^3$ of solution.
starch indicator

(a)

Method

Fill the burette with FA 1.
Pipette $25.0\text{ cm}^3$ of FA 4 into a conical flask.
Use the measuring cylinder to add $10\text{ cm}^3$ of FA 2 to the same conical flask.
Use the measuring cylinder to add $10\text{ cm}^3$ of FA 3 to the same conical flask. The mixture will become brown because of the formation of $\text{I}_2$ , and will become cloudy because of the formation of the white precipitate of $\text{CuI}$ .
Add FA 1 from the burette until the mixture becomes a light brown colour.
Add 10 to 20 drops of starch indicator until the mixture becomes blue-black.
Continue to titrate with FA 1 until the blue-black colour disappears leaving a mixture with an off-white solid. This is the end-point.
You should test that the end-point has been reached by adding 2 more drops of starch indicator. If the titration has reached the end-point the added starch indicator will cause no change in colour.
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 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 1 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations.
Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FA 4 required ............................. $\text{cm}^3$ of FA 1.

(c)

Calculations

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 in the volume of FA 1 calculated in (b).

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

(ii)

Using the equations on page 2, calculate the number of moles of copper(II) ions in $25.0\text{ cm}^3$ of FA 4.

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

(iii)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of copper(II) ions in FA 4.

concentration of $\text{Cu}^{2+}$ in FA 4 = ............................. $\text{mol dm}^{-3}$

(iv)

Calculate the value of $x$ in $\text{CuSO}_4 \cdot x\text{H}_2\text{O}$ .

$x$ = .............................

(d)

(i)

Calculate the maximum percentage error in one of your accurate titres.

maximum percentage error = ............................. %

(ii)

A student suggests that the experiment could be made more accurate if the volume of FA 3 was measured using a burette.

Give a reason why the student might make this suggestion.

Explain why this change would not improve the accuracy of the experiment.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis13M

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