2019 NJC Prelim P4 ANS

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, w here 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 g iven 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 cm 3 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 consistent results. Record your titration results in the space below. Results Temperature of the solution in the beaker = ………….. oC 1 2 Final burette reading / cm3 Initial burette reading / cm3 Volume of FA 2 used / cm3 [1]Record the temperature of the solution in the beaker [1] Records all burette readings to 2 d.p including a(ii). (MMO) [1] Has at least 2 titre values ± 0.10 cm3 (MMO) [1] Table has appropriate headings and units (PDO) [-1] Incorrect calculation for volume of FA2 [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. Average vol of FA 2 used = (x.00 +.00)/2 =x.00 cm3 (round to 2 d.p.) [1] * Do not award marks if inconsistent values used * Do penalise if no working 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 b(ii), and hence determine the molar concentration of IO3─ ions in FA 6. Amt of S2O32− = (ans from b(ii)/1000) x 0.05 = A mol [1] Amt of IO3─ = = B mol [IO3─] = = c mol dm–3 [1] (Allow ecf) 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. [Ca2+]= ½ [IO3─] = ½ c mol dm–3 [1] (Allow ecf) Ksp = [Ca2+(aq)] [IO3─(aq)]2 = [½ C] [C]2 = ½ C3 mol3 dm–9 [1] (Allow ecf) (Do not award the mark if the units is wrong) concentration of Ca2+ ions in FA 6 = …………………………….. solubility product of Ca(IO3)2 = …………………………….. [2] (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? To prevent the dilution of the saturated FA 6 solution which results in lesser amount of …………………………………………………………………………………………………………………. IO3─ reacted during titration and hence smaller titre value [1] (Do not award mark if there is no mention to the change in the titre value or change in conc of FA 6 or IO3─.) [1]

5 NJC SH2 Preliminary Examination 9729/04/19 [Turn over (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 K sp value that you have calculated and suggest an improvement that might allow a value closer to the literature value to be obtained. If the calculated Ksp > literature Ksp value, the explanation is as follow: The experiment is not carried out at 20 oC [1]. Ca(IO3)2 (s) ⇌ Ca2+ (aq) + 2IO3─ (aq) Hence, the equilibrium position shifted to the right. Use a thermostatically controlled water bath maintained at 20 oC during the preparation of FA 6 before carrying out the filtration [1]. or If the calculated Ksp < literature Ksp value, the explanation is as follow: The salt has not dissolve to form a saturated solution or reach equilibrium with its