ACJC Prelim P4_Answers

3 © ACJC 2018 9729/04/Prelim/2018 [Turn over Answer all the questions in the spaces provided. 1 Determination of the value of n in IOn− IOn− is an oxoanion of iodine. It can react with iodide ions, I−, in an acidic medium, to produce only molecular iodine and water. This amount of iodine can be analysed via titration with sodium thiosulfate, Na 2S2O3. 2S2O32− + I2 S4O62− + 2I− In this question, you will perform an experiment to determine the stoichiometric ratio between IOn− and I2 and the value of n in IOn−. FA 1 is solid hydrated sodium thiosulfate, Na2S2O3•5H2O FA 2 is a solution containing 0.00825 mol dm−3 KIOn FA 3 is 1.0 mol dm−3 sulfuric acid, H2SO4 FA 4 is 0.25 mol dm−3 potassium iodide, KI You are also provided with starch solution. (a) Preparation of a standard solution of 0.0484 mol dm −3 sodium thiosulfate (i) Determine the mass of FA 1 required to prepare 250 cm 3 of sodium thiosulfate solution with a concentration 0.0484 mol dm−3. [Ar: S, 32.1; O, 16.0; Na, 23.0; H, 1.0] Amount of Na2S2O3 = 0.0484 1000 × 250 = 0.0121 mol Mr of Na2S2O3•5H2O = 248.2 Mass of Na2S2O3•5H2O = 0.0121 × 248.2 = 3.003 = 3.00 g mass of FA 1 required = ………………….. g [1] You will follow the instructions to perform the preparation of the standard solution. Record your measurements in Table 1.1. (ii) 1. Using a mass balance, weigh out accurately the mass of FA 1 determined in (a)(i) into a weighing bottle. 2. Dissolve FA 1 in about 50 cm 3 of deionised water in a 100 cm3 beaker. 3. Transfer the solution and washings into a 250 cm 3 volumetric flask, using a filter funnel. 4. Top up to the 250 cm 3 mark with deionised water. 5. Stopper the flask and shake to ensure solution is homogeneous. Label this solution as FA 5. Complete Table 1.1 with your mass measurements and calculate the mass of FA 1 that you used. Table 1.1 Mass of weighing bottle and FA 1 / g Mass of empty weighing bottle / g

4 © ACJC 2018 9729/04/Prelim/2018 [Turn over Mass of FA 1 / g 3.00 [2] Records all masses to same number of decimal places (PDO) Records correctly all masses (PDO) Mass of FA 1 measured is ± 0.05 g of that determined in (a)(i) (MMO) (b) Preparation and titration of the reaction mixture 1. Fill a burette with FA 5. 2. Using a 25.0 cm 3 pipette, add 25 cm3 of FA 2 to a 250 cm3 conical flask. 3. Using a measuring cylinder, add 10.0 cm 3 of FA 3, followed by 10.0cm3 of FA 4 to the same conical flask and swirl quickly. 4. Immediately titrate the I2 in the conical flask with FA 5. When a pale yellow colour is obtained, add 1 cm 3 of starch solution. The end-point is reached when the blue – black colour of the solution decolourises. 5. Repeat steps 2 to 4 until consis