RVHS Prelim P4 Soln

River Valley High School Pg 1 JC 2 H2 Chemistry 9729 2021 Prelim Practical Examination RIVER VALLEY HIGH SCHOOL JC 2 PRELIM PRACTICAL EXAMINATION H2 CHEMISTRY 9729 Paper 4 19 AUGUST 2021 2 HOURS 30 MINUTES NAME CLASS INDEX NO. INSTRUCTIONS TO CANDIDATES DO NOT OPEN THIS BOOKLET UNTIL YOU ARE TOLD TO DO SO. Read these notes carefully. Write your name, class and index number in the spaces at the top of this page. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a 2B pencil for any diagrams or graph. 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. Shift Laboratory For Examiner’s Use total 55 _________________________________________________________________________________ This Question Paper consists of 20 printed pages and 1 blank page. Answer all the questions in the spaces provided.

River Valley High School Pg 2 JC 2 H2 Chemistry 9729 2021 Prelim Practical Examination 1 Determine the solubility product, Ksp, of calcium iodate(V), Ca(IO3)2 For Examiner’s Use Calcium iodate is used in the manufacture of disinfectants, antiseptics, and deodorants. Its solubility in water is low. When calcium iodate(V) is mixed with water, the following equilibrium is established. Ca(IO3)2(s) = Ca2+(aq) + 2IO3-(aq) The total amount of IO3-(aq) in the saturated salt solution is determined using iodometry. Excess potassium iodide, KI, is added to an acidified solution of the filtrate, liberating iodine. Reaction 1 IO3-(aq) + 5I-(aq) + 6H+(aq) ® 3I2(aq) + 3H2O(l) The liberated iodine is then titrated with a standard solution of sodium thiosulfate. Reaction 2 2S2O32-(aq) + I2(aq) ® 2I-(aq) + S4O62-(aq) (a) You are provided with FA 1 a saturated solution of Ca(IO3)2 in KIO3(aq) FA 2 0.200 mol dm-3 sodium thiosulfate, Na2S2O3 FA 3 aqueous solution of potassium iodide, KI FA 4 dilute sulfuric acid, H2SO4 Starch indicator Titration of filtrate, FA 1 1. Pipette 25.0 cm3 of FA 1 into a conical flask. 2. Using a measuring cylinder, add about 10 cm3 of FA 4 to the conical flask. 3. Using another measuring cylinder, add about 10 cm3 of FA 3 to the conical flask. 4. Add FA 2 from the burette into the conical flask until a pale yellow solution is obtained. 5. Add about 5 drops of starch indicator and continue adding FA 2 until the blue-black colour just disappears. 6. Record your titration results, to an appropriate level of precision, in the space on Page 3. 7. Repeat the titration as many times as necessary until consistent results are obtained.

River Valley High School Pg 3 JC 2 H2 Chemistry 9729 2021 Prelim Practical Examination Results Titration 1 2 Final burette reading / cm3 18.95 18.95 Initial burette reading/ cm3 0.00 0.00 Volume of FA 2 used / cm3 18.95 18.95 Supervisor’s mean 18.95 Student’s mean Difference 1 2 3 4 (b) (i) From your titrations, obtain a suitable volume of FA 2 (VFA 2) to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 2 used = 18.95 cm3 5 (ii) Use the volume of FA 2 obtained in (b)(i) to calculate the amount of IO3-(aq) present in 25.0 cm3 of FA 1. Amount of S2O32- = = 0.00374 mol IO 3- º 3I2 º 6S2O32- amount of IO3- in 25.0 cm3 of FA 1 = 0.00374 ÷ 6 = 6.32 ´ 10-4 mol 6 18.950.200 1000 ´

River Valley High School Pg 4 JC 2 H2 Chemistry 9729 2021 Prelim Practical Examination (iii) Calculate the concentration of IO3-(aq) in FA 1. Concentration of dissolved IO3- in FA 1 = = 2.52 ´ 10-2 mol dm− 3 7 (iv) Calculate the concentration of Ca2+(aq) in FA 1. Concentration of Ca2+ = !.#! ´ %&!" ! = 0.0126 mol dm− 3 8 (c) Write an expression for the solubility product, Ksp, of calcium iodate(V) and using your answer in parts (b)(iii) and (b)(iv), calculate a value for the solubility product, Ksp, of Ca(IO3)2, giving its units in your answer. Ksp = [Ca2+][IO3-]2 = (0.0126)( 2.52 ´ 10-2)2 = 8.02 ´ 10-6 mol3 dm-9 9 10 11 (d) Another student prepared a solution of FA 1 and performed the titration. He obtained a value for the solubility product, Ksp, of 7.96 ´ 10-6. A literature value for this solubility product is 6.47 ´ 10-6 at 25 °C. You should assume that apparatus of the same precision was used in both cases. State a possible reason for the higher value of Ksp obtained by the student. The student performed the experiment at a temperature higher than 25 °C. Hence, the position of equilibrium for Ca(IO3)2(s) = Ca2+(aq) + 2IO3-(aq) shifted to the right. 12 4 10006.32 10 25.0 - ´´

River Valley High School Pg 5 JC 2 H2 Chemistry 9729 2021 Prelim Practical Examination Planning For Examiner’s Use The solubility of calcium iodate(V), at a particular temperature, can be defined as: the mass of calcium iodate(V) that will dissolve in and just saturate 1000 cm3 of solvent at that temperature. A saturated solution is one in which no more solid can dissolve at a particular temperature. In a saturated solution with undissolved solid, the following equilibrium is established. Ca(IO3)2(s) = Ca2+(aq) + 2IO3-(aq) Like most salts, solubility of calcium iodate(V) increases when the temperature of the solution increases. (e) Plan a procedure to determine the solubility of calcium iodate(V) in water. You may assume that you are provided with: • deionised water • solid calcium iodate(V), Ca(IO3)2 • filter funnel and filter paper • water bath • thermometer • the apparatus and equipment normally found in a school or college laboratory. Your plan should include • calculation of the mass (approximate) of Ca(IO3)2 that will dissolve in 100 cm3 of deionised water to give a saturated solution [molar mass of Ca(IO3)2 = 390 g mol-1; solubility of Ca(IO3)2 at 20 °C » 6.15 ´ 10-3 mol dm-3] • practical details of how you would - prepare a saturated solution in 100 cm3 of deionised water in a 250 cm3 conical flask, - maintain the temperature of the mixture, - separate the saturated solution from the undissolved solid, - obtain the mass of dry solid, - ensure that an accurate and reliable value of solubility of calcium iodate(V) in water is obtained. • details of how the results would then be used to obtain the solubility of Ca(IO3)2 in water at 20 °C.

River Valley High School Pg 6 JC 2 H2 Chemistry 9729 2021 Prelim Practical Examination Approximate mass of Ca(IO3)2 that can dissolve in 100 cm3 of deionised water = 6.15 ´ 10-3 ´ 390 ´ %&&%&&& = 0.240 g 1. Measure 100.0 cm3 of deionised water using a 100.0 cm3 measuring cylinder and add it to a 250 cm3 conical flask. Place the conical flask in a thermostatically-controlled water bath at 20 °C. Place a thermometer into the flask to monitor the temperature of the water. 2. Weight accurately the mass of 1.0 g of Ca(IO3)2 in a weighing bottle. Using a spatula, add small amounts of pre-weighed Ca(IO3)2 to the deionised water in the conical flask and stir continuously the mixture with a glass rod. Continue adding Ca(IO3)2, with stirring, until some undissolved solid remains. Allow 10 min for the mixture to reach equilibrium. 3. Using an electronic balance, weigh accurately a dry filter paper. Record the mass as m1. Using a dry filter funnel and the same dry filter paper, filter the mixture immediately. 4. Place the filter paper and its contents from (3) under an infra-red lamp for 30 mins. 5. Cool and weigh the filter paper and its contents. 6. Repeat the dry-cool-weigh process from (4) and (5) until the difference in mass is ≤ ±0.05g. Record the final mass of residue and filter paper as m2. 7. Mass of Ca(I