2021 SAJC Prelim P4 Answer

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 16 Aug 2021 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes Shift Laboratory For Examiner’s Use 1 2 3 4 Total This document consists of 21 printed pages including this page. READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. 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. The number of marks is given in the brackets [ ] at the end of each question or part question. 13 18 10 14 55

2 [Turn Over 1. Determination of the value of x in the oxyanion of iodine, IOx– Iodine is able to form more than one oxyanion, IOx–, polyatomic ions that contain oxygen, each containing a different number of oxygen atoms. In this experiment, you will determine the value of x in the oxyanion of iodine, IOx–. You will first react IOx– ions with an excess of iodide ions, I–, to form iodine, I2 as shown in equation 1. equation 1 IOx– + y I– + z H+ → !"𝒚$ I2 + 𝒛$ H2O where x, y and z are all integers The amount of iodine produced will then be determined by titration with thiosulfate ions, S2O32–. I2 + 2S2O32– → 2I– + S4O62– FA 1 is a solution containing 0.0150 mol dm–3 IOx– ions. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is 1.00 mol dm–3 potassium iodide, KI. FA 4 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Procedure 1. Fill the burette with FA 4. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Use a measuring cylinder to add 25 cm3 of FA 2 to the conical flask. 4. Use another measuring cylinder to add 10 cm3 of FA 3 to the conical flask. The solution will turn brown as iodine is produced. 5. Add FA 4 from the burette until the solution in the conical flask turns yellow. 6. Add 5 drops of starch indicator to the conical flask. The solution will turn blue-black. 7. Continue to add more FA 4 from the burette until the blue-black colour just disappears. This is the end-point of the titration. 8. Record your titration results, to an appropriate level of precision, in the space provided on page 3. 9. Repeat points 2 to 7 until consistent results are obtained.

3 [Turn Over Results Initial burette reading /cm3 0.00 0.00 Final burette reading /cm3 25.60 25.70 Volume of FA 4 /cm3 25.60 25.70 • Correct headers, units and final burette reading greater than initial burette reading • Correct precision to 2 d.p • 2 consistent values within ±0.10 cm3