CJC 2022 JC2 H2 Chem Prelim P4 QP 0308

[Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical 23 August 2022 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name and class in the boxes above. 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. Instruction Catholic Junior College JC2 Preliminary Examinations Higher 2 Shift Laboratory For Examiner’s Use 1 / 15 2 / 13 3 / 27 Total / 55

2 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 Answer all the questions in the spaces provided. 1 Determination of the Mr of a hydrated ethanedioate salt, using acidified potassium manganate(VII) by titration Calcium ethanedioate is the major component of the most common type of human kidney stones. It is one of a series of salts formed from ethanedioic acid, H 2C2O4. Another of these salts can be represented by the formula X2C2O4•H2O, where X is a Group 1 metal. Solution Q contains 64.5 g dm–3 of X2C2O4•H2O in deionised water. You are not provided with Q. FA 1 is a diluted solution of Q, in which 35.70 cm 3 of Q was made up to 250 cm 3 with deionised water in a graduated flask. FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. In this question, you will perform a titration. The data from this titration will be used to determine: • the concentration of C2O42– ions in Q. • the Mr of X2C2O4•H2O, and hence the identity of the metal X. (a) Titration of FA 1 against FA 2 In this titration, FA 2 is run from a burette into the conical flask containing FA 1 and FA 3. Initially, the colour of the FA 2 will take some time to disappear. After some FA 2 has been added, sufficient Mn 2+(aq) ions will be present to allow the reaction to occur faster. The end point is reached when a permanent pale pink colour is obtained. (i) 1. Fill the burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 1 into the conical flask. 3. Using an appropriate measuring cylinder, transfer 25.0 cm3 of FA 3 to the same conical flask. 4. Heat this solution to about 60 oC. 5. Run FA 2 from the burette into this flask until a permanent pale pink colour is obtained. Be careful when you titrate under the hot condition. 6. Record your titration results to an appropriate level of precision, in the space provided next page. 7. Repeat points 1 to 6 until consistent results are obtained. 8. Turn off your Bunsen burner.

3 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 Titration results [5] (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = …………………………..[1] (iii) The equation for the reaction between ethanedioate ions , C 2O42– and manganate(VII) ions is shown below. 5C2O42–(aq) + 2MnO4–(aq) + 16H+(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g) Calculate the amount of ethanedioate ions, C2O42– in 25.0 cm3 of FA 1. amount of C2O42– in in 25.0 cm3 of FA 1 = …………………………..[2] (iv) Determine the concentration in mol dm–3, of C2O42– in Q. concentration of C2O42– in Q = …………………………..[2]

4 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 (v) Use your answer to (a)(iv) to calculate the Mr of the ethanedioate salt. Mr of the ethanedioate salt = …………………………..[1] (vi) Hence, deduce the identity of X. Show your working clearly. [Ar: C, 12.0; O, 16.0; H, 1.0; Li, 6.9; Na, 23.0; K, 39.1; Rb, 85.5; Cs, 132.9; Fr, 223.0] X is …………………………..[2] (b) A student performed the experiment in (a)(i) using a sample of another ethanedioate salt. The student obtained a mean titre value of 22.20 cm3. The teacher calculated that the volume of FA 2 required should be 22.40 cm 3. The teacher told the student that the total percentage error from the apparatus in the experiment was 0.4%. Calculate the error in the student’s result, based on these data. Sta te and explain whether or not the student’s result is accurate. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. …………………………………………………………………………………………….[2] [Total: 15]

5 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 2 Investigation of reaction between manganate(VII) ions and ethanedioate ions FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4 (same as in Q1). FA 3 is 1.0 mol dm–3 sulfuric acid, H2SO4 (same as in Q1) . FA 4 is 0.200 mol dm–3 ethanedioic acid, H2C2O4. FA 5 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 6 is 0.100 mol dm–3 potassium iodide, KI. You are also provided with a starch indicator. Acidified potassium manganate(V II) oxidises ethanedioate ions, C 2O42− as shown in reaction 1 . The Mn 2+ ions produced in reaction 1 act as a catalyst for the reaction. This is an example of ‘autocatalysis’. 2MnO4–(aq) + 5C2O42−(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l) reaction 1 You are to investigate how the rate of reaction changes with [MnO4−]. To do this, you will prepare a reaction mixture containing FA 2, FA 3 and FA 4. At timed intervals, you will withdraw five aliquots (portions) of the reaction mixture, add them to 10 cm 3 of excess KI which will “quench” the reaction by reacting away all MnO 4– via a redox reaction as shown in reaction 2. 2MnO4–(aq) + 10I−(aq) + 16H+(aq) → 2Mn2+(aq) + 5I2(aq) + 8H2O(l) reaction 2 You will then titrate the iodine produced in the resulting solutions against sodium thiosulfate (reaction 3). 2S2O32−(aq) + I2(aq) → 2I−(aq) + S4O62−(aq) reaction 3 Your titre values will indicate the concentration of MnO4− remaining in the reaction mixture at different times. Hence, the rate of reaction between MnO4− and C2O42− at different times can be determined by graphical analysis of your results. You should read all of the instructions on this page and the method on the next page before you start this exper