AS & A Level CHEMISTRY9701/33 Paper 3 Advanced Practical Skills 1 May/June 2023 Authentic Marking Scheme

Cambridge International AS & A Level
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CHEMISTRY 9701/33
Paper 3 Advanced Practical Skills 1 May/June 2023

2 hours

You must answer on the question paper.

You will need: The materials and apparatus listed in the confidential instructions
Insert (enclosed)

INSTRUCTIONS
● Answer all questions.
● Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs.
● Write your name, centre number and candidate number in the boxes at the top of the page.
● Write your answer to each question in the space provided.
● Do not use an erasable pen or correction fluid.
● Do not write on any bar codes.
● You may use a calculator.
● You should show all your working and use appropriate units.

Session
INFORMATION
● The total mark for this paper is 40.
● The number of marks for each question or part question is shown in Laboratory
brackets [ ].
● The Periodic Table is printed in the question paper.
● Important values, constants and standards are printed in the
question paper.
● Notes for use in qualitative analysis are provided in the For Examiner’s Use
question paper.
● The insert contains additional resources referred to in the questions. 1

2

3

Total

This document has 12 pages.


DC (RW) 302724/4
© UCLES 2023 [Turn over

, 2

Quantitative Analysis

Read through the whole method before starting any practical work. Where appropriate, prepare a table
for your results in the space provided.

Show the precision of the apparatus you used in the data you record.

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


1 A redox reaction takes place between hydroxylamine, NH2OH, and the iron(III) ion, Fe3+, in
acidic conditions. The iron(III) ion is reduced to an iron(II) ion, Fe2+. The reaction is slow at room
temperature but is complete in a few minutes at 100 °C. The reaction is shown by one of the
following equations.

equation 1 NH2OH(aq) + Fe3+(aq) Fe2+(aq) + H+(aq) + ½N2(g) + H2O(l)

equation 2 NH2OH(aq) + 2Fe3+(aq) 2Fe2+(aq) + ½N2O(g) + 2H+(aq) + ½H2O(l)

equation 3 NH2OH(aq) + 3Fe3+(aq) 3Fe2+(aq) + NO(g) + 3H+(aq)

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.

MnO4–(aq) + 5Fe2+(aq) + 8H+(aq) Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)

FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4.
FA 2 is a solution prepared by boiling a 1.00 dm3 aqueous mixture containing 3.30 g of
hydroxylamine hydrochloride, NH2OH•HCl , excess iron(III) chloride, 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 cm3 of FA 2 into a conical flask.
• Use the 25 cm3 measuring cylinder to add 10 cm3 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 .............................. cm3.

• 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).

© UCLES 2023 9701/33/M/J/23

, 3

Results
I

II

III

IV

V

VI

VII

[7]


(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 cm3 of FA 2 required .............................. cm3 of FA 1. [1]


(c) Calculations

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


amount of KMnO4 = .............................. mol [1]

(ii) Use your answer to (c)(i) to calculate the amount, in mol, of iron(II) ions in 25.0 cm3 of
solution FA 2.


amount of Fe2+ = .............................. mol [1]

(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 NH2OH•HCl = .............................. mol [2]

(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 ................. . [1]

[Total: 13]
© UCLES 2023 9701/33/M/J/23 [Turn over

, 4

2 The reaction between thiosulfate ions and hydrogen ions produces a precipitate of sulfur. When
the concentration of hydrogen ions, [H+(aq)], is kept constant, the rate is proportional to one of the
following:

• the concentration of thiosulfate ions, [S2O32–(aq)]
• the square of the concentration of thiosulfate ions, [S2O32–(aq)]2.

You will determine which relationship is correct by mixing solutions of sodium thiosulfate and
sulfuric acid. You will measure the time taken for a fixed amount of sulfur to be precipitated.

Throughout these experiments care must be taken to avoid inhaling any SO2 gas that is
produced. It is very important that as soon as each experiment is complete, the contents of
the beaker are emptied into the quenching bath and the beaker is rinsed thoroughly.

FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4.
FA 4 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3.

(a) Method

Experiment 1

• Use the 50 cm3 measuring cylinder to transfer 50.0 cm3 of FA 4 into the 100 cm3 beaker.
• Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 3.
• Add FA 3 to FA 4 in the beaker and start timing immediately.
• Stir the mixture once and place the beaker on the printed insert.
• View the printing on the insert from above, through the solution.
• Stop timing when the print on the insert becomes obscured.
• Record this reaction time to the nearest second in Table 2.1.
• Empty the contents of the beaker into the quenching bath.
• Rinse and dry the beaker and glass rod so they are ready for use in Experiment 2.

Experiment 2

• Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 4 into the 100 cm3 beaker.
• Use the same measuring cylinder to add 20.0 cm3 of distilled water to the same beaker.
• Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 3.
• Add FA 3 to the mixture of FA 4 and distilled water in the beaker and start timing
immediately.
• Stir the mixture once and place the beaker on the printed insert.
• View the printing on the insert from above, through the solution.
• Stop timing when the print on the insert becomes obscured.
• Record this reaction time to the nearest second in Table 2.1.
• Empty the contents of the beaker into the quenching bath.
• Rinse and dry the beaker and glass rod so they are ready for use in Experiment 3.

Experiment 3

• Carry out one further experiment to investigate how the reaction time changes with a
different volume of FA 4.

Do not use a volume of FA 4 that is less than 20.0 cm3.

Record your results in Table 2.1.




© UCLES 2023 9701/33/M/J/23