Chemistry 9701/33 — May/June 2023

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

A redox reaction takes place between hydroxylamine, $\text{NH}_2\text{OH}$ , and the iron(III) ion, $\text{Fe}^{3+}$ , in acidic conditions. The iron(III) ion is reduced to an iron(II) ion, $\text{Fe}^{2+}$ . The reaction is slow at room temperature but is complete in a few minutes at $100\text{ }^\circ\text{C}$ . The reaction is shown by one of the following equations.

\begin{aligned} \text{equation 1} \quad &\text{NH}_2\text{OH(aq)} + \text{Fe}^{3+}\text{(aq)} \rightarrow \text{Fe}^{2+}\text{(aq)} + \text{H}^+\text{(aq)} + \frac{1}{2}\text{N}_2\text{(g)} + \text{H}_2\text{O(l)} \\ \text{equation 2} \quad &\text{NH}_2\text{OH(aq)} + 2\text{Fe}^{3+}\text{(aq)} \rightarrow 2\text{Fe}^{2+}\text{(aq)} + \frac{1}{2}\text{N}_2\text{O(g)} + 2\text{H}^+\text{(aq)} + \frac{1}{2}\text{H}_2\text{O(l)} \\ \text{equation 3} \quad &\text{NH}_2\text{OH(aq)} + 3\text{Fe}^{3+}\text{(aq)} \rightarrow 3\text{Fe}^{2+}\text{(aq)} + \text{NO(g)} + 3\text{H}^+\text{(aq)} \end{aligned}

You will carry out a titration to determine which of equations 1, 2 or 3 best represents the reaction. The iron(II) ions formed in the reaction with the hydroxylamine are oxidised by manganate(VII) ions.

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

FA 1 is $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII), $\text{KMnO}_4$ .
FA 2 is a solution prepared by boiling a $1.00\text{ dm}^3$ aqueous mixture containing $3.30\text{ g}$ of hydroxylamine hydrochloride, $\text{NH}_2\text{OH}\cdot\text{HCl}$ , excess iron(III) chloride, $\text{FeCl}_3$ , and excess sulfuric acid. Any water lost by evaporation was replaced after cooling.
FA 3 is dilute sulfuric acid.

Assume that one mole of hydroxylamine hydrochloride gives one mole of hydroxylamine in solution.

(a)

Method

Fill the burette with FA 1.
Pipette $25.0\text{ cm}^3$ of FA 2 into a conical flask.
Use the $25\text{ cm}^3$ measuring cylinder to add $10\text{ cm}^3$ of FA 3 into the same conical flask.
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, all your burette readings and the volume of FA 1 added in each accurate titration.

Keep FA 3 for use in Question 2(a).

(b)

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

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

(c)

Calculations

(i)

Calculate the amount, in mol, of potassium manganate(VII) present in the volume of FA 1 in (b).

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

(ii)

Use your answer to (c)(i) to calculate the amount, in mol, of iron(II) ions in $25.0\text{ cm}^3$ of solution FA 2.

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

(iii)

Calculate the amount, in mol, of hydroxylamine hydrochloride that has reacted in the FA 2 pipetted into the conical flask. Show your working.

amount of $\text{NH}_2\text{OH}\cdot\text{HCl}$ = .............................. mol

(iv)

Use your answer to (c)(iii) to deduce which of the three suggested equations corresponds to your results. Show your working.

The correct equation number is ................. .

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation15M

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