ASR 2020 J2Prelim H2Chem P4 QP

ASRJC JC2 PRELIM 2020 9729/04/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2020 JC 2 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 20 /____ CHEMISTRY 9729/04 Paper 4 Practical 27 August 2020 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. 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 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 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. Quantitative Analysis Notes are printed on pages 19 and 20. Shift Laboratory At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 14 2 / 18 3 / 11 4 / 12 Total / 55 This document consists of 20 printed pages.

2 ASRJC JC2 PRELIM 2020 9729/04/H2 Answer all the questions in the spaces provided. 1 Determination of the oxidation number of iodine in a compound In this experiment you will determine the oxidation number of iodine in one of its compounds by titration. You are provided with the following: FA 1 is a 0.0197 mol dm–3 solution of the iodine–containing compound. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous potassium iodide, KI. FA 4 is 0.105 mol dm–3 sodium thiosulfate, Na2S2O3. (This solution is also used in Question 2.) starch indicator FA 1 reacts with excess acidified potassium iodide to produce iodine, I2. This iodine is then titrated with aqueous sodium thiosulfate using starch indicator. (a) Titration of iodine formed against FA 4 1. Fill the burette with FA 4. (You will use the same burette for Question 2.) 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Using a 25 cm3 measuring cylinder, add 10 cm3 of FA 2 to the same conical flask. 4. Using the same measuring cylinder, add 20 cm 3 of FA 3 to the mixture in the conical flask. The mixture will now be a red–brown colour, due to iodine produced. 5. Add FA 4 from the burette until the mixture becomes light brown. 6. Then add 10 drops of starch indicator. The mixture will change to a dark blue colour. 7. Continue titrating until the mixture becomes colourless. This is the end–point. 8. Carry out as many accurate titrations as you think necessary to obtain consistent results. 9. Record in a suitable form below all of your burette readings and the volume of FA 4 added in each accurate titratio