Chemistry 9701/31 — May/June 2021

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

Iron(II) sulfate crystals, $\text{FeSO}_4\cdot x\text{H}_2\text{O}$ , contain water of crystallisation. You will carry out a titration to determine the value of $x$ in the formula, where $x$ is an integer. A solution containing a known mass of the crystals will be titrated with acidified aqueous potassium manganate(VII) of known concentration.

5\text{Fe}^{2+}(\text{aq}) + \text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) \rightarrow 5\text{Fe}^{3+}(\text{aq}) + \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})

FA 1 contains $26.52\text{ g dm}^{-3}$ of hydrated iron(II) sulfate, $\text{FeSO}_4\cdot x\text{H}_2\text{O}$ .
FA 2 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 3 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Fill the burette with FA 2.
Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Use the $25\text{ cm}^3$ measuring cylinder to transfer $25\text{ cm}^3$ of FA 3 into the same conical flask.
Carry out 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 accuracy 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.

Keep FA 1 for use in Question 3.

(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 FA 1 required .............................. $\text{cm}^3$ of FA 2.

(c)

Calculations

(i)

Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures.

(ii)

Calculate the number of moles of potassium manganate(VII) present in the volume of FA 2 calculated in (b).

moles of $\text{KMnO}_4$ = .............................. mol

(iii)

Calculate the number of moles of iron(II) sulfate present in $1.00\text{ dm}^3$ of FA 1.

moles of $\text{FeSO}_4$ = .............................. mol

(iv)

Calculate the mass of iron(II) sulfate present in $1.00\text{ dm}^3$ of FA 1.

mass of $\text{FeSO}_4$ = .............................. g

(v)

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

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

(d)

Iron(II) sulfate in solution is readily oxidised by air to form iron(III) sulfate.

State the effect, on the value of $x$ calculated in (c)(v), if some of your sample of FA 1 had oxidised before you carried out the titration.
Explain your answer.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation8M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations16M

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