2021 TJC H2Chemistry P4 Ans and Prep List Prelim

1 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2021 CANDIDATE NAME CIVICS GROUP / CENTRE NO. / INDEX NO. / CHEMISTRY 9729/04 Paper 4 Practical 26 August 2021 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 19 and 20. 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 20 printed pages. For Examiner’s Use 1 / 12 2 / 17 3 / 14 4 / 12 Total / 55 Shift Laboratory 2021 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 / 2021 Answer all the questions in the spaces provided. 1 Determination of water of crystallisation in a hydrated iron(III) salt A variety of hydrated iron(III) sulfates are known. Solutions of iron(III) sulfate are used in dyeing as a mordant, and as a coagulant for industrial wastes. FA 1 is hydrated iron(III) sulfate with formula Fe2(SO4)3.nH2O. The addition of excess zinc to a solution of FA 1 reduces the Fe3+ ions to Fe2+ ions. The amount of Fe2+ ions can be determined quantitatively by 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 are to prepare a standard solution using FA 1 and perform titrations to determine the value of n, the water of crystallisation in FA 1. You are provided with FA 2, dilute sulfuric acid, FA 3, 0.0200 mol dm-3 potassium manganate(VII), KMnO4, zinc powder. (a) Preparation of standard solution of hydrated iron(III) salt 1. Weigh accurately about 6.5 g of the FA 1 provided using the weighing bottle. Transfer all the solid into a 250 cm3 beaker. Determine the mass of solid FA 1 used and record all your weighings, to an appropriate level of precision, in the space below. Mass of empty weighing bottle / g 4.78 Mass of weighing bottle and FA 1 / g 11.28 Mass of weighing bottle and residual FA 1 / g 4.78 Mass of FA 1 used / g 6.50 [1] • Tables have correct headers and units including correct subtraction 2. Use a measuring cylinder to add about 100 cm3 of FA 2 to the beaker. Stir the mixture with a glass rod for 3 minutes. Ignore any cloudiness that remains. 3. Transfer the solution into a 250 cm3 volumetric flask. Rinse the beaker with deionised water and pour the washings into the volumetric flask. 4. Make up to the 250 cm3 mark with deionised water, stopper and mix thoroughly by inverting the flask a number of times. 5. Label this solution FA 4.

3 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2021 (b) Preparation of Fe2+ solution from FA 4 6. Use a measuring cylinder to transfer 100 cm3 of FA 4 into a 250 cm3 beaker. 7. Add all the zinc powder into the beaker. Cover the beaker with a white tile. 8. Allow the reaction to take place for about 5 minutes, stirring the reaction mixture from time to time. 9. Filter the mixture into the dry conical flask provided using dry filter paper and filter funnel. Ignore any reaction that may still be taking place. 10. Label the filtrate as FA 5. Proceed to 1(c) once you have collected sufficient filtrate. (c) (i) Titration of FA 5 against FA 3 11. Fill a burette with FA 3. 12. Use a pipette to transfer 10.0 cm3 of FA 5 into a 250 cm3 conical flask. 13. Use a measuring cylinder to add about 10 cm3 of FA 2 to this flask. 14. Titrate FA 5 with FA 3 from the burette until the appearance of the first permanent pale-pink colour. 15. Record your titration results, to an appropriate level of precision, in the space below. 16. Repeat steps 12 to 15 until consistent results are obtained. Titration Results Final burette reading / cm3 9.20 9.20 Initial burette reading / cm3 0.00 0.00 Volume of FA 3 used / cm3 9.20 9.20 Ö Ö • Tables have correct headers and units Note: Mark is lost if any final and initial burette readings are inverted or 50 is used as the initial burette reading. • All mass reading in (a) are recorded to 2 or 3 dp and burette readings & volume used for all accurate titres in the titration table are recorded to 2dp. • At least two uncorrected titres for end-point within ±0.10 cm3. [3]

4 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2021 (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 + 9.202 = • 9.20 cm3 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 Supervisor’s VFA 4 / mFA 3 = 1.42 Difference between student’s and supervisor’s VFA 4 / mFA 3 2 marks: If difference is £ 0.045 cm3 g-1 1 mark: If difference is > 0.045 but £ 0.075 cm3 g-1 0 mark: For a difference > 0.075 VFA 3 = ………………………………………………[3] (d) (i) Calculate the amount of Fe2+ in 10.0 cm3 of FA 5. 5Fe2+ Ξ MnO4- No. of moles of Fe2+ = 9.20/1000 ´ 0.020 ´ 5 = 9.20 x 10-4 mol amount of Fe2+ in 10.0 cm3 of FA 5 = …………………………………………. [1] (ii) In step 7, an excess of zinc was added to convert the Fe3+ to Fe2+. Calculate the amount of Fe3+ in 250 cm3 of FA 4. No. of moles of Fe3+ = 9.20 x 10-4 x 25010 = 0.0230 mol amount of Fe3+ in 250 cm3 of FA 4 = …………………………………………. [1]

5 [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2021 (iii) Use your answer from 1(d)(ii) to calculate the Mr of the hydrated iron(III) sulfate, Fe2(SO4)3.nH2O, in FA 1. Fe2(SO4)3.nH2O Ξ 2Fe3+ No. of moles of Fe2(SO4)3.nH2O = 0.0230 / 2 mol = 0.0115 mol Mr of Fe2(SO4)3.nH2O = 6.50 / 0.0115 = 565.2 Mr of the hydrated iron(III) sulfate = …………………………………. Hence, deduce the value of n, the water of crystallisation in the hydrated iron(III) sulfate. [Ar: Fe, 55.8; S, 32.1; O, 16.0; H, 1.0] n = {565.2 - [2(55.8) + 3(32.1) + 12(16.0)]} ÷ 18.0 = 9 (nearest whole number) n = ……………………………….. [2] (e) In step 9, excess zinc was filtered off before titration of FA 5 against FA 3 to avoid any possible reaction between zinc and FA 3. Suggest another reason why it was necessary to filter off the excess zinc metal, and what effect failing to do it would have on the titre values. [1] • Zinc metal that is not removed will continue to reduce Fe3+ formed during the titration to Fe2+, resulting in a higher than expected titre. [Tota