2021 RI H2 Chem Prelims P4 Questions

1 © Raffles Institution 2021 9729/04/A/21 [Turn Over CANDIDATE NAME ( ) CLASS 21S0 RAFFLES INSTITUTION 2021 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY Paper 4 Practical 9729/04 25 August 2021 2 hours 30 minutes Do NOT turn over the Question Booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST. Write your name and class on the space provided when instructed to do so. Give details of the practical shift and laboratory where appropriate, in the space provided. Write in dark blue or black pen. You may use a 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 number of marks is given in brackets [ ] at the end of each question or part question. 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 23 and 24. Shift Laboratory Bench Number For Examiner’s Use Question Marks 1 / 16 2 / 11 3 / 14 4 / 14 Total / 55 This Question Paper consists of 23 printed pages and 1 blank page.

2 © Raffles Institution 2021 9729/04/A/21 1 Determination of the solubility product of calcium iodate(V) Calcium iodate(V), Ca(IO3)2, has low solub ility in water. When a sample of this salt is mixed with water, a small amount dissolves and the following equilibrium is established in a saturated solution of Ca(IO3)2. Ca(IO3)2(s) ⇌ Ca2+(aq) + 2IO3–(aq) You are to perform an experiment to determine the solubility product, Ksp, of Ca(IO3)2(s). When a sample of solid Ca(IO3)2 is added to a mixture containing specific volumes of aqueous potassium iodate(V), KIO3(aq), and aqueous potassium nitrate, KNO3(aq), both of known concentrations, a small amount of solid dissolves and the above equilibrium is established after some time. The mixture is then filtered and the amount of iodate(V) ions, IO3, in the filtrate is determined as described below.  Excess potassium iodide is added to an acidified solution of the filtrate containing IO3 ions and iodine is liberated, as shown in equation 1. equation 1 IO3–(aq) + 5I–(aq) + 6H+(aq)  3I2(aq) + 3H2O(l)  The liberated iodine is then titrated with a standard solution of sodium thiosulfate, as shown in equation 2. equation 2 2S2O32–(aq) + I2(aq)  2I–(aq) + S4O62–(aq) (a) Preparation of a saturated solution of Ca(IO3)2 in KIO3(aq) and KNO3(aq), FA 1 You do not need to perform this part of the experiment. 1. Use a measuring cylinder to transfer 50.0 cm3 of 0.0100 mol dm3 KIO3(aq) to a clean and dry beaker. 2. Use a nother measuring cylinder to transfer 50.0 cm 3 of 0.0100 mol dm 3 KNO3(aq) to the beaker. 3. Add 3 spatulas of solid Ca(IO3)2 to the beaker. 4. Stir the mixture thoroughly. Leave the mixture to stand for several minutes to allow equilibrium to be reached. 5. Filter the mixture through a dry filter paper into a dry conical flask to collect the filtrate, FA 1, for titration in (b).

3 © Raffles Institution 2021 9729/04/A/21 [Turn Over FA 1 is the filtrate from (a), a saturated solution of Ca(IO3)2 in KIO3(aq) and KNO3(aq). FA 2 is 0.100 mol dm3 sodium thiosulfate, Na2S2O3. FA 3 is an aqueous solution of potassium iodide, KI. FA 4 is 1.00 mol dm3 dilute sulfuric acid, H2SO4. Starch indicator (b) Titration of filtrate, FA 1 1. Fill a burette with FA 2. 2. Pipette 25.0 cm3 of FA 1 into a 250 cm3 conical flask. 3. Use a measuring cylinder to add about 10 cm3 of FA 3 to the conical flask. 4. Use another measuring cylinder to add about 10 cm3 of FA 4 to the conical flask. 5. Run FA 2 from the burette into th e conical flask until a pale yellow solution is obtained. 6. Add 5 drops of starch solution to the conical flask. Continue adding FA 2 until the blue-black colour just disappears. 7. Record your titration results, to an appropriate level of precision, in the space provided below. 8. Perform sufficient titrations until consistent results are obtained. (i) Titration results [4] (ii) From your titration results, 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]

4 © Raffles Institution 2021 9729/04/A/21 (c) Calculations (i) Calculate the amount of S2O32– ions present in the volume of FA 2 recorded in (b)(ii). amount of S2O32– ions = …………………...................... [1] (ii) Calculate the amount of IO3– ions present in 25.0 cm3 of the filtrate, FA 1. amount of IO3– ions in 25.0 cm3 of FA 1 = …………………...................... [1] (iii) Calculate the amount of IO3– ions present in the saturated solution prepared in (a). amount of IO3– ions in the saturated solution = ………………….................... [1] (iv) Calculate the amount of IO3– ions from the Ca(IO3)2 that has dissolved. amount of IO3– ions from the Ca(IO3)2 that has dissolved = …………………...... [1]

5 © Raffles Institution 2021 9729/04/A/21 [Turn Over (v) Calculate the amount of Ca 2+ ions present in the saturated solution prepared in (a). amount of Ca2+ ions in the saturated solution = ………………........................ [1] (vi) Write an expression for the solubility product, Ksp, of calcium iodate( V). Include units in your answer. [1] (vii) Calculate a value for the solubility product, Ksp, of calcium iodate(V). Ksp = ………………....................... [1] (d) In (a) step 5, dry filter paper and dry conical flask are used in the preparation of FA 1. Suggest and explain the likely consequence on the titre volume if these instructions were not followed. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ………………………………………………………………………………………………….[1]

6 © Raffles Institution 2021 9729/04/A/21 (e) A student claims that the Ksp of calcium iodate(V) is affected by the volume of KIO3(aq). To investigate, the student repeated the procedure in (a) using four different volumes of KIO3(aq) as shown in Table 1.1. Table 1.1 Experiment 1 2 3 4 Volume of 0.0100 mol dm3 KIO3 / cm3 0 25 75 100 Volume of 0.0100 mol dm3 KNO3 / cm3 100 75 25 0 Titration of each collected filtrate was also carried out following the procedure in (b). (i) The total concentration of ions in a solution is known to affect the solubility product and the solubility of a salt. Hence, KNO 3(aq) is added to keep the total concentration of ions constant. With the use of relevant calculations, show that the total concentration of ions prepared in Experiments 1 and 2 are the same. [1]

7 © Raffles Institution 2021 9729/04/A/21 [Turn Over (ii) Predict and explain how increasing the volume of KIO3(aq) used will a