2022 TJC Prelims H2 Paper 4 Qns and Ans

1 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2022 CANDIDATE NAME CIVICS GROUP / CENTRE NO. / INDEX NO. / CHEMISTRY 9729/04 Paper 4 Practical 29 August 2022 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your Civics Group and name on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 23 and 24. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 22 printed pages and 2 blank pages. For Examiner’s Use 1 / 12 2 / 19 3 / 14 4 / 10 Total / 55 Shift Laboratory 2022 JC2 PRELIMINARY EXAMINATIONS HIGHER 2

2 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2022 Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of water of crystallisation in FeSO4·nH2O Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided. Show your working and appropriate significant figures in the final answer to each step of your calculations. The amount of Fe2+ ions can be determined quantitatively by a titration against a standard solution of potassium manganate(VII), KMnO4. The reaction is shown below. 5Fe2+ + MnO4− + 8H+ → 5Fe3+ + Mn2+ + 4H2O In this experiment, you will determine the percentage by mass of water of crystallisation in FeSO4·nH2O. You will titrate a solution of FA 1 against FA 3. You are provided with FA 1, contains 26.07 g dm–3 of hydrated iron(II) sulfate, FeSO4•nH2O, where n is an integer. FA 2, 1.0 mol dm-3 dilute sulfuric acid, H2SO4. FA 3, 0.0200 mol dm−3 potassium manganate(VII), KMnO4, You are to keep FA 2 and FA 3 for questions 2 and 3. (a) (i) Titration of FA 1 against FA 3 1. Fill a burette, labelled FA 3, with FA 3. 2. Use a pipette to transfer 10.0 cm3 of FA 1 into a 250 cm3 conical flask. 3. Use a measuring cylinder to add about 10 cm3 of FA 2 to this flask. 4. Titrate FA 1 with FA 3 from the burette until the appearance of the first permanent pale-pink colour. 5. Record your titration results, to an appropriate level of precision, in the space on Page 3. 6. Repeat steps 1 to 4 until consistent results are obtained.

3 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2022 Titration Results Final burette reading / cm3 9.20 9.30 Initial burette reading / cm3 0.00 0.00 Volume of FA 3 used / cm3 9.20 9.30   Tables have correct headers and units [1] Note: Mark is lost if any final and initial burette readings are inverted or 50 is used as the initial burette reading. All burette readings & volume used for all accurate titres in the titration table are recorded to 2dp and at least two uncorrected titres for end-point within ±0.10 cm3. [1] [2] (ii) From your titrations, obtain a suitable volume of FA 3 , VFA 3, to be used in your calculations. Show clearly how you obtained this volume. Average titre = 9.20 + 9.30 2 = 9.25 cm3 [1] Answer should be recorded to 2.d.p Mark is lost if there are arithmetic errors in the table. Mark is lost if the titres used are not identified either in the table (by, for example, a tick) or in a calculation. Accuracy 2 marks: If difference is  0.20cm3 1 mark: If difference is  0.40 cm3 0 mark: For a difference > 0.40 cm3 Volume of FA 3 = ………………………………………………[3] (b) (i) Calculate the amount of Fe2+ in 10.0 cm3 of FA 1. 5Fe2+ Ξ MnO4− No. of moles of Fe2+ = 9.25/1000  0.020  5 = • 9.25 x 10-4 mol [1] amount of Fe2+ in 10.0 cm3 of FA 1 = …………………. [1]

4 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2022 (ii) Calculate the amount of Fe2+ in 1 dm3 of FA 1. No. of moles of Fe2+ = 9.25 x 10-4 x 1000 𝟏𝟎 = • 0.0925 mol [1] amount of Fe2+ in 1 dm3 of FA 1 = …………………. [1] (iii) Calculate the Mr of the hydrated iron(II) sulfate, FeSO4.nH2O, in FA 1. Mr of Fe(SO4).nH2O = 26.07 / 0.0925 = 281.8 [1] Mr of the hydrated iron(II) sulfate = …………………………………. Hence, deduce the value of n, the water of crystallisation in the hydrated iron(II) sulfate. [Ar: Fe, 55.8; S, 32.1; O, 16.0; H, 1.0] n = 281.8 − [(55.8) + (32.1) + 4(16.0)]  18.0 = 7.22 = 7 (nearest whole number) [1] Note: This answer must be a positive integer. n = ……………………………….. [2] (iv) Hence, determine the percentage by mass of water of crystallisation in FeSO4·nH2O. Show your working. percentage by mass of water of crystallisation in FeSO4·nH2O. = 𝟕.𝟐𝟐×𝟏𝟖 𝟐𝟖𝟏.𝟖 x100% = 46.1% [1] Note for marker: If student uses the rounded off integer, need to re-calculate the Mr. otherwise, penalise. Percentage by mass of water of crystallisation = ………………………………..

5 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2022 [1] (v) Iron(II) sulfate in solution is readily oxidised by air to form iron(III) sulfate. State the effect, on the value of n calculated in (b)(iii), if some of your sample of FA 1 had oxidised before you carried out the titration. Explain your answer. Amount /Volume of MnO 4- used is smaller OR mass/amount of Fe 2+ used is smaller [1] mass/moles/amount water larger so (mole) ratio larger so n increases [1] Allow difference in titre/moles of MnO4− / Fe2+ will be too small to change the (integer) value of n for 1 mark. Allow for 1 mark: less MnO4− / Fe2+ and n increases [2] [Total: 12] 2 To investigate the reaction between manganate(VII) ions and C2O42− ions. In this experiment, you will investigate the kinetics of the reaction between MnO4– and C2O42–. FA 4 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3 FA 5 is 0.200 mol dm-3 sodium ethanedioate, Na2C2O4 FA 6 is 0.100 mol dm–3 potassium iodide, KI FA 2 and FA 3 are solutions from question 1. You are to keep FA 5 for question 3. You are also provided with a starch indicator. Acidified potassium manganate(VII) ions oxidises C 2O42− ions to produce Mn 2+ ions which act as an autocatalyst for the reaction. 2 MnO4–(aq) + 5 C2O42−(aq) + 16 H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(I) Acidified potassium manganate(VII) ions also oxidises I− ions according to the following reaction: 2MnO4–(aq) + 10I−(aq) + 16H+(aq) → 2Mn2+(aq) + 5I2(aq) + 8H2O(I) You will add a solution of acidified potassium manganate(VII) to a mixture containing FA 2 and FA 5. At timed intervals, you will draw 10 cm 3 aliquots of the reaction mixture. The

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