TJC 2024 JC2 Prelims H2 Paper 4 Solutions

9729 / TJC Prelim / 2024 [Turn Over TEMASEK JUNIOR COLLEGE 2024 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME MARK SCHEME CLASS CENTRE NUMBER S INDEX NUMBER Chemistry 9729/04 Paper 4 Practical 26 August 2024 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your name, CG, centre number 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. This document consists of 18 printed pages and 2 blank pages. For Examiner’s Use 1 / 16 2 / 13 3 / 16 4 / 10 Total / 55 Shift Laboratory

2 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2024 Answer all the questions in the spaces provided. 1 Determination of the formula of hydrated sodium thiosulfate. In this experiment, you will determine the value of x in the formula of hydrated sodium thiosulfate, Na2S2O3xH2O, where x is an integer. You will first react IO3– ions with an excess of iodide ions, I–, to form iodine, I2. The amount of iodine produced will then be determined by titration with thiosulfate ions, S2O32–. I2 + 2S2O32– → 2I– + S4O62– FA 1 is 27.3 g dm-3 of hydrated sodium thiosulfate, Na2S2O3xH2O. FA 2 is 0.0150 mol dm-3 potassium iodate(V), KIO3. FA 3 is 0.500 mol dm-3 potassium iodide, KI. FA 4 is 2.20 mol dm-3 hydrochloric acid, HCl. Starch indicator (a)(i) Procedure 1. Fill the burette with FA 1. 2. Pipette 25.0 cm3 of FA 2 into a conical flask. 3. Use a measuring cylinder to add 10 cm3 of FA 3 to the conical flask. 4. Use another measuring cylinder to add 20 cm3 of FA 4 to the conical flask. The solution will turn brown. 5. Add FA 1 from the burette until the solution in the conical flask turns pale yellow. 6. Add 10 drops of starch indicator to the conical flask. The solution will turn blue-black. 7. Continue to add more FA 1 from the burette until the blue-black colour just disappears. This is the end point of the titration. 8. Repeat steps 2 to 7 until consistent results are obtained. Keep FA 4 for use in Question 2. Keep FA 1 and FA 3 for use in Question 3. Titration results Final burette reading/ cm3 20.50 41.00 Initial burette reading/ cm3 0.00 20.50 Volume of FA 1/ cm3 20.50 20.50 Correct headers with units [1] All burette readings to 0.05 cm3 [1] Correct calculation of titre volumes. At least 2 uncorrected titres to 0.10 cm3. [1] [3]

3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2024 [Turn over (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 1 used = 320.50 + 20.50 = 20.50 cm2 Use consistent readings to correctly calculate volume of FA 1. Working must be shown or ticks put next to the two (or more) accurate titres selected. [1] Accuracy [2] 2 marks if difference is  0.20 cm3 1 mark if difference is  0.40 cm3 0 mark if difference is  0.40 cm3 Volume of FA 1 = ..........................cm3 [3] (b) (i) The equation for this reaction between iodate(V) and iodide is shown. IO3– + 5I– + 6H+ → 3I2 + 3H2O Calculate the amount of iodine that form s when 25.0 cm 3 of FA 2 reacts with 10 cm3 of FA 3. [1] amount of IO3– in FA 2 = 425 0.0150 3.75 10 mol1000 − =  amount of I– in FA 3 = 310 0.500 5.00 10 mol1000 − =  IO3– is the limiting reagent. IO3–  3I2 amount of I2 formed = 43.75 10 3− = 31.125 10 mol− = 31.13 10 mol− [1] (allow 3 or 4 s.f.) (ii) Use your titration results and the equation on page 2 to calculate the concentration of Na2S2O3 in FA 1 in mol dm-3. [2] I2  2 S2O32– amount of Na2S2O3 = -3 -32 1.13 10 2.26 10 mol  =  [1] conc of Na2S2O3 in FA 1 = 3 -32.26 10 0.110 mol dm20.50 1000 − = [1]

4 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2024 (iii) Hence, determine the value of x in the formula of hydrated sodium thiosulfate, Na2S2O3xH2O, used to prepare FA 1. [Ar: Na, 23.0; S, 32.1; O, 16.0; H, 1.0] Mr of Na2S2O3.xH2O = 27.3 248.20.110 = [1] 2(23.0) + 2(32.1) + 3(16.0) + x(18.0) = 248.2 x = 5 [1] Value of x = .......... [5] Clear and logical working shown from (b)(i) to (iii) [1] All units shown from (b)(i) to (iii) [1] 2 d.p. for (a)(ii), 3 or 4 s.f. for (b)(i), (b)(ii), integer for x in (b)(iii) [1] Any working not attempted loses these marks. (c) (i) Calculate the maximum percentage error in the volume of FA 1 used. error for each burette reading = ± 0.05 cm3 Maximum percentage error = ± 2x0.05 20.50 x 100 = ± 0.488% [1] Maximum percentage error = ......................% [1] (ii) A student suggested that a more accurate value of x could be obtained if a 10.0 cm3 pipette is used to measure FA 3 rather than the measuring cylinder. State whether you agree with the student. Explain your answer. Do not agree with the student as FA 3 (KI) is in excess. An accurate measurement of FA 3 volume does not affect the amount of iodine, the titre volume of S2O3- and the calculation of x value. [1] [Total: 16]

5 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2024 [Turn over 2 In this experiment you will use a thermometric method to determine the concentration of a sample of alkali. You will mix varying volumes of acid with a fixed volume of the alkali and measure the temperature rises that occur. You will use your experimental data to calculate the enthalpy change for the neutralisation of the acid with alkali. FA 4 is 2.20 mol dm–3 hydrochloric acid, HCl. FA 5 is aqueous sodium hydroxide, NaOH. (a) Procedure 1. Use the thermometer to measure the initial temperature of FA 5. initial temperature of FA 5 = 28.6 °C 2. Place a polystyrene cup inside a second polystyrene cup and place both cups in a 250 cm3 beaker. 3. For Experiment 1, use a measuring cylinder to transfer 20 cm3 of FA 5 into the cup. 4. Using a second measuring cylinder, measure 18 cm3 of water and add 2 cm3 of FA 4 to the same measuring cylinder. 5. Transfer the solution in the second measuring cylinder into the cup containing FA 5. 6. Stir the mixture in the cup gently with the thermometer. Read and record the maximum temperature obtained. 7. Empty, rinse and shake dry the cup ready for use in Experiment 2. 8. Repeat the procedure using 20 cm 3 of FA 5 for each experiment and the volumes of water and FA 4 shown in table 2.1. In each case, measure and record the maximum temperature. Table 2.1 Experiment Volume of water / cm3 Volume of FA 4 / cm3 Maximum temperature / 0C 1 18.0 2.0 30.0 2 14.0 6.0 32.6 3 10.0 10.0 34.8 4 6.0 14.0 34.8 5 2.0 18.0 34.8 6 8.0 12.0 34.9 Carry out one further experiment which will enable you to determine more precisely the volume of FA 4 that gives the highest maximum temperature. Record your measurements for all experiments in Table 2.1. [3] [1] Six thermometer readings recorded