2021 ACJC H2 Chem Prelim P4 Ans for sharing with other schools

Name Index Number Form Class Tutorial Class Subject Tutor ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/04 Higher 2 Paper 4 Practical 3 August 2021 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS CAREFULLY Write your name, class and index number 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. For Examiner’s Use Question No. Marks 1 (15 m) 2 (23 m) 3 (17 m) Presentation of answers TOTAL (55 m) This document consists of 19 printed pages and 1 blank page. 9729/04/Prelim/2021 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2021 Department of Chemistry [Turn over Shift Laboratory

© ACJC 2021 9729/04/Prelim/2021 [Turn over 2 1 Determination of the total concentration of ethanedioate ions with sodium hydroxide and potassium manganate(VII) (a) Procedure FA 1 is an aqueous mixture of ethanedioic acid, H 2C2O4, and sodium ethanedioate, Na2C2O4 FA 2 is 0.100 mol dm–3 sodium hydroxide, NaOH FA 3 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4 You are advised to read parts (a)(i) and (a)(ii) before starting practical work and to carry out a ROUGH titration until the relevant colour change is seen in each part of the titration. (i) Determine by titration, with sodium hydroxide, the concentration, in mol dm–3, of the ethanedioic acid in FA 1.  Fill the burette with FA 2.  Pipette 25.0 cm3 of FA 1 into a conical flask and add to the flask a few drops of thymol blue indicator.  Run FA 2 from the burette into the conical flask until the solution turns green (just before it turns blue).  Repeat the titration as many times as necessary to provide reliable results.  Tabulate your titration results in the space provided. (ii) Determine by titration, with potassium manganate( VII), the total concentration, in mol dm–3, of ethanedioate ions in FA 1.  Fill the second burette with FA 3.  Pipette 25.0 cm 3 of FA 1 into a conical flask. Use the measuring cylinder provided to add to the flask 25 cm 3 of 1 mol dm –3 sulfuric acid, 10 cm 3 of 0.01 mol dm–3 manganese(II) sulfate and 40 cm3 of distilled water.  Put the thermometer in the conical flask and heat the solution u ntil the temperature is just over 65 oC.  Carefully remove the thermometer and place the hot flask under the burette.  If the neck of the flask is too hot to hold safely, use a folded paper towel to hold the flask. Run in 1 cm3 of FA 3 and swirl the flask until the colour of the potassium manganate(VII) has disappeared then continue the titration as normal until a permanent pale pink colour is obtained. This is the end-point. - If a brown colour appears during titration , reheat the flask to 65 oC. The brown colour should disappear and the titration can be completed as above. - If the brown colour does not disappear on reheating , discard the solution and start the titration again.  Repeat the titration as many times as necessary to provide reliable results.  Tabulate your titration results in the space provided.

© ACJC 2021 9729/04/Prelim/2021 [Turn over 3 (b) Results (i) Tabulate your titration results from (a)(i). Make certain your recorded results show the precision of your practical work. Final burette reading / cm3 Initial burette reading / cm3 19.10 0.00 38.10 19.10 Volume of FA 2 used / cm3 19.10 19.00 (ii) From your titration results in (b)(i), obtain a suitable volume of FA 2, VFA 2, to be used in your calculations. Show clearly how you obtained this volume. Average titre = (19.10 + 19.00) / 2 = 19.05 cm3 VFA 2 = ....................................... (iii) Tabulate your titration results from (a)(ii). Make certain your recorded results show the precision of your practical work. Final burette reading / cm3 Initial burette reading / cm3 28.00 0.00 28.00 0.00 Volume of FA 3 used / cm3 28.00 28.00 (iv) From your titration results in (b)(iii), obtain a suitable volume of FA 3, VFA 3, to be used in your calculations. Show clearly how you obtained this volume. Average titre = (28.00 + 28.00) / 2 = 28.00 cm3 VFA 3 = ....................................... [5]

© ACJC 2021 9729/04/Prelim/2021 [Turn over 4 (c) Calculations (i) Calculate the amount of NaOH used in (a)(i). Amount of NaOH = 0.100 x 19.05 x 10–3 = 1.91 x 10–3 mol amount of NaOH = ....................................... [1] (ii) Use the titration data from (a)(i) to calculate the concentration, in mol dm–3, of H2C2O4 in FA 1. H2C2O4 + 2NaOH → Na2C2O4 + 2H2O Amount of C2O42– = 1.91 x 10–3 / 2 = 9.55 x10–4 mol [H2C2O4] = 9.55 x 10–4 / 25 x 10–3 = 0.0382 mol dm–3 [H2C2O4] in FA1 = ....................................... [1] (iii) Calculate the amount of KMnO4 added from the burette in (a)(ii). Amount of KMnO4 = 28.00 x 10–3 x 0.0200 = 5.60 x 10–4 mol amount of KMnO4 = ………………………………… [1] (iv) Use titration data from (a)(ii) to calculate the total concentration, in mol dm–3, of C2O42– ions in FA 1. MnO4– + 8H+ + 5e– → Mn2+ + 4H2O C2O42– → 2CO2 + 2e– 2MnO4– ≡ 5 C2O42– Amount of C2O42– = (5.60 x 10–4 / 2 ) x 5 = 1.40 x 10–3 mol [C2O42–] = 1.40 x10–3 / 25 x10–3 = 0.0560 mol dm–3 [C2O42–] in FA1 = ………………………………… [1]

© ACJC 2021 9729/04/Prelim/2021 [Turn over 5 (v) Use your answers to (c)(ii) and (c)(iv) to calculate the concentration, in g dm–3, for each of the following: [Mr of H2C2O4 = 90.0 ; Mr of Na2C2O4 = 134.0]  H2C2O4 [H2C2O4] = 0.0382 x 90.0 = 3.44 g dm–3 [H2C2O4] in FA1 = ………………………………… [1]  Na2C2O4 Amount of C2O42– from Na2C2O4 in 1 dm3 solution = 0.0560 – 0.0382 = 0.0178 mol [Na2C2O4] = (0.0178 x 134.0) = 2.39 g dm–3 [Na2C2O4] in FA1 = ………………………………… [1]

© ACJC 2021 9729/04/Prelim/2021 [Turn over 6 (d) In a kinetics experiment, 50 cm 3 of ethanedioic acid with 10 cm 3 of sulfuric acid and 40 cm3 of deionised water were added in a 250 cm3 conical flask. 25 cm3 of potassium manganate(VII) solution was added to the conical flask and the start time recorded. Small aliquots from the mixture were drawn and added to a conical flask containing potassium iodide at regular time intervals. The conical flask was then titrated with aqueous sodium thiosulfate from a burette. The diagram shows the sketch for the graph of volume of sodium thiosulfate against time for the experiment. Describe and explain any unusual features in the graph. …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... ……………………………………………………………………………………………….. [4] [Total: 15] volume of Na2S2O3 The initial gradient of the graph was a gentle negative slope, but it became a steeper negative gradient and ended with a gentle negative slope.