ACJC_H2_CHEM_Chem_P4 Ans Prelim

2 © ACJC 2017 9729/04/Prelim/2017 [Turn over Question 1 In this experiment, you are required to find the concentration in mol dm -3 of sodium thiosulfate, Na2S2O3, in solution FA 1. You are provided with: FA 1 contains sodium thiosulfate, Na2S2O3. FA 2 is potassium manganate (VII) containing 1.83 g dm-3 KMnO4. FA 3 is 1 mol dm-3 sulfuric acid, H2SO4. FA 4 is 10% potassium iodide containing 4 g dm-3 KI. You are also provided with starch indicator. For Examiner’s Use Dilution of FA 2 (a) By using a burette measure between 41.00 cm3 and 42.00 cm3 of FA 2 into the 250 cm3 graduated flask. Record your burette readings and the volume of FA 2 added to the flask in the space below. Final burette reading / cm3 41.50 Initial burette reading / cm3 0.00 Volume of FA2 used / cm3 41.50 [2] Make up the contents of the flask to the 250 cm 3 mark with distilled water. Stopper and mix the contents thoroughly by slowly inverting the flask a number of times. Label this solution FA5. Titration Filll a second burette with FA 1, the solution containing sodium thiosulfate. Use a measuring cylinder to transfer 10 cm 3 of FA 3 and 10 cm 3 of FA 4 into a conical flask. Pipette 25.0 cm 3 of FA 5 into the conical flask containing the mixture of FA 3 and FA 4. The potassium manganate (VII) oxidises potassium iodide to iodine, I2. Titrate the liberated iodine with FA 1 as follows. Run the solution from the burette into the conical flask until the initial brown colour of the iodine becomes pale yellow. Then add 1 cm 3 of the starch indicator and continue to add FA 1 drop by drop until the blue - black colour of the starch-iodine complex disappears, leaving a colourless solution. This is the end-point of the titration. Record your titration results in the space below. Make certain that your recorded results show the precision of your working. Repeat your experiment to obtain consistent results. 1 2 Final burette reading / cm3 24.20 24.20 Initial burette reading / cm3 0.00 0.00 Volume of FA1 used / cm3 24.20 24.20 [6]

3 © ACJC 2017 9729/04/Prelim/2017 [Turn over (b) From your titration results obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Average titre = (24.20 + 24.20) / 2 = 24.20 cm3 Volume of FA1 = ………………… [1] For Examiner’s Use Calculations Show your working and appropriate significant figures in all of your calculations. (c) Calculate how many moles of KMnO 4 are contained in the FA 2 run into the graduated flas