2019 NJC Prelim P4 QP

NJC SH2 Preliminary Examination 9729/04/19 [Turn over NATIONAL JUNIOR COLLEGE SH 2 Year  End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidate answer on the Question paper. 9729/04 Tuesday 20 August 2019 2 hours 30 minutes READ THE INSTRUCTIONS FIRST Write your identification number and name. Circle 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, highlighters, 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, submit the question paper. The number of marks is given in brackets [ ] at the end of each question or part question. Shift 1 2 3 Laboratory CM41 CM42 CM43 CM44 PH31 PH32 PH33 BI23 BI24 For Examiner’s Use 1 / 12 2 / 21 3 / 14 4 / 8 Total / 55 This document consists of 19 printed pages and 1 blank page.

2 NJC SH2 Preliminary Examination 9729/04/19 1 Determination of the solubility product of calcium iodate(V) When solid calcium iodate( V) is added to water to form a saturated solution, an equilibrium between the undissolved salt and its aqueous ions is established. Ca(IO3)2 (s) Ca2+ (aq) + 2IO3 ─ (aq) The solubility product, Ksp , of calcium iodate(V) is expressed as follows. Ksp = [Ca2+(aq)] [IO3─(aq)]2 The solubility product can be found by determining the equilibrium concentration of IO3─ ions in a saturated solution of calcium iodate(V) through a redox titration. Iodate(V) ions, IO3─, react with iodide ions according to the following equation. IO3─ (aq) + 5I⁻(aq) + 6H+ (aq) 3I2 (aq) + 3H2O (l) The iodine produced in this reaction may be titrated against sodium thiosulfate using starch solution as indicator. Thiosulfate ions react with iodine according to the following equation: 2S2O32–(aq) + I2(aq)  S4O62–(aq) + 2I–(aq) In this question, you will prepare a saturated solution of calcium iodate(V) to carry out the redox titration. Using the experimental results, you will determine the solubility product of calcium iodate(V). You are provided with: FA 1 is solid calcium iodate(V), Ca(IO3)2 FA 2 is 0.05 mol dm3 sodium thiosulfate, Na2S2O3 FA 3 is 0.50 mol dm3 potassium iodide, KI FA 4 is 0.50 mol dm3 sulfuric acid, H2SO4 FA 5 is starch indicator Preparation of a saturated solution of calcium iodate (V), FA 6 (a) Procedure 1. Use a 100 cm 3 measuring cylinder to transfer 80.0 cm3 of deionised water into a 250 cm3 beaker. 2. Transfer all FA 1 from the weighing bottle into the beaker. Stir for 1 to 2 minutes and leave to stand for 5 minutes. There will be some Ca(IO3)2 solids left undissolved. 3. Use a 0.2 oC division thermometer, record the temperature of the solution in the beaker. 4. To remove the undissolved solids, filter the saturated solution into another clean and dry 250 cm 3 beaker using a dry filter paper and dry funnel. Label this solution as FA 6.

3 NJC SH2 Preliminary Examination 9729/04/19 [Turn over Titration (b) Procedure (i) 1. Fill a burette with FA 2. 2. Pipette 10.0 cm3 of FA 6 into a conical flask. 3. Use a 10 cm3 measuring cylinder to add 5.0 cm 3 of FA 3, followed by 5.0 cm 3 of FA 4 to the same conical flask and swirl quickly. 4. Titrate the iodine in the conical flask with FA 2 until the brown colour of the iodine becomes pale yellow. 5. Add about 10 drops of starch solution to the flask and continue adding FA 2 until the blue-black colour just disappears. 6. Repeat the titration as many times as you think necessary to obtain consis tent results. Record your titration results in the space below. Results [4] (ii) From your titres, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. 10.0 cm3 of FA 6 produced an amount of iodine which required ……....… cm3 of FA 2. [1]

4 NJC SH2 Preliminary Examination 9729/04/19 (c) Calculations Show your working and appropriate significant figures in the final answer to each part. (i) Calculate the number of moles of S2O32− in the volume of FA 2 recorded in 1b(ii), and hence determine the molar concentration of IO3─ ions in FA 6. number of moles of S2O32− =…………………………….. concentration of IO3─ ions in FA 6 =…………………………….. [2] (ii) Determine the molar concentration of Ca2+ in FA 6 and hence, calculate the solubility product, Ksp, of calcium iodate(V), stating the units. concentration of Ca2+ ions in FA 6 = …………………………….. solubility product of Ca(IO3)2 = …………………………….. [2]

5 NJC SH2 Preliminary Examination 9729/04/19 [Turn over (d) Explain why it was necessary to use the dry apparatus (e.g. beaker, filter paper and funnel) in the preparation of FA 6, and what effect failing to do it would have on the titre values? …………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………[1] (e) A literature value for the solubility product of calcium iodate( V) is found to be 6.15 × 106 at 20 °C. State a possible reason for the difference in the Ksp value that you have calculated and suggest an improvement that might allow a value close r to the literature value to be obtained. …………………………………………………………………………………………………………………. …………………………………………………………………………………………………………………. …………………………………………………………………………………………………………………. …………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………[2] [Total: 12]

6 NJC SH2 Preliminary Examination 9729/04/19 BLANK PAGE

7 NJC SH2 Preliminary Examination 9729/04/19 [Turn over 2 Determination of the enthalpy change of neutralisation, Hn FA 7 is a solution of HCl of unknown concentration FA 8 is a solution of 1.0 mol dm−3 of NaOH In this question, you will carry out a series of experiments where different volumes of FA 7 and FA 8 are mixed together. You will determine the temperature change of the reaction mixture, T, of each experiment and then analyse your results graphically in order to determine the  concentration of HCl in FA 7  maximum temperature change, Tmax  value for the enthalpy change of neutralisation, Hn (a) Determining the change in temperature for a series of reactions between FA 7 and FA 8 (i) Experiment 1 1. Place a Styrofoam cup inside a second Styrofoam cup held in a 250 cm 3 beaker to prevent it from tipping over. 2. Using a measuring cylinder, transfer 20.0 cm3 of FA 7 into the Styrofoam cup. 3. Measure the temperature of FA 7 in the Styrofoam cup. Record the initial temperature of the solution of FA 7 as TFA 7. 4. Rinse and dry the thermometer. 5. Measure 50.0 cm 3 of FA 8 using another measuring cylinder.