Chemistry 9701/33 — October/November 2022

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

An iron compound can contain $\text{Fe}^{2+}$ ions, $\text{Fe}^{3+}$ ions or both ions.

In this experiment, you will determine the percentage by mass of iron in FA 1, an unknown compound. You will first prepare a solution of the compound and then carry out a titration using acidified potassium manganate(VII), $\text{KMnO}_4$ .

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})

The end-point of the titration is when all the $\text{Fe}^{2+}$ ions have been oxidised and so unreacted potassium manganate(VII) causes the colour of the solution to become a permanent pale pink.

FA 1 is a sample of the unknown compound.
FA 2 is $0.0100\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 3 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .

(a)

Method

Weighing the compound

Weigh the sample of FA 1 and its container.
Tip the FA 1 into the $250\text{ cm}^3$ beaker.
Reweigh the container including any residual FA 1.
Record both your readings clearly in the space below.
Calculate the mass of FA 1 transferred into the beaker. Record the mass.

Preparing the solution

Add approximately $200\text{ cm}^3$ of distilled water to the beaker and stir until the FA 1 has dissolved.
Pour the contents carefully into the $250\text{ cm}^3$ volumetric flask.
Rinse the contents of the beaker with a little distilled water and add these washings to the flask.
Fill the flask to the line with distilled water and shake thoroughly.
Label this solution FA 4.

Titration

Fill the burette with FA 2.
Pipette $25.0\text{ cm}^3$ of FA 4 into a conical flask.
Use the $25\text{ cm}^3$ measuring cylinder to add $15\text{ cm}^3$ of FA 3 to the conical flask.
Add FA 2 from the burette until the solution in the flask turns a permanent pink.
Carry out a rough titration and record your burette readings in the space below.

\text{rough titre} = \text{.............................. cm}^3

Carry out as many accurate titrations as you think are necessary to obtain consistent results.
Make sure that your recorded results show the precision of your practical work.
Record in a suitable format, in the space below, all your burette readings and the volume of FA 2 added in each accurate titration.

(b)

From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtained this value.

25.0\text{ cm}^3\text{ of FA 4 required .............................. cm}^3\text{ of FA 2.}

(c)

Calculations

(i)

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

(ii)

Calculate the amount, in mol, of manganate(VII) ions in the volume recorded in (b).

\text{amount of } \text{MnO}_4^- = \text{.............................. mol}

(iii)

Calculate the amount, in mol, of iron(II) ions in the weighed sample of FA 1.

\text{amount of } \text{Fe}^{2+} = \text{.............................. mol}

(iv)

Calculate the percentage by mass of iron in FA 1. Show your working.

\text{percentage by mass of iron} = \text{.............................. \%}

(d)

A student carries out the same experiment as in (a). The student receives a sample of FA 1 in a container with a lid. The student records the initial mass of the container with its lid and the sample of FA 1. Then the student records the mass of the container with the residue but forgets to replace the lid.

How would this error alter the student’s answer to (c)(iv)?
Explain your answer.

(e)

State two assumptions that have been made in calculating the percentage by mass of iron in FA 1 in (c)(iv).

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation10M
Q3Qualitative Analysis · Manipulation, Measurement and Observation13M

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