NJC Prelim P4_Solutions

1 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over 2018 SH2 H2 Chemistry P4 Solutions Answer all the questions in the spaces provided. 1 Determination of the average relative formula mass of a mixture of two copper salts In this experiment, you will determine the aver age relative formula mass of a mixture of two copper salts by titration. A solution of the copper salts mixture reacts with excess acidified potassium iodide, producing iodine. 2Cu2+(aq) + 4I–(aq) 2Cu I(s) + I2(aq) This iodine is then titrated with aqueous sodium thiosulfate, using starch indicator. FA 1 is an aqueous solution of the copper salt prepared by dissolving 26.0  g of the salt mixture to make 1.00 dm3 of solution. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous potassium iodide, KI. FA 4 is 1.50 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Preparation of diluted FA 4 1. Pipette 25.0  cm3 of FA 4 into the 250 cm3 graduated flask. 2. Make up the contents of the flask to the 250 cm 3 mark with deionised water. 3. Stopper the flask and mix the contents thoroughly to ensure a homogeneous solution. This prepared solution is diluted FA 4. Titration 1. Fill the burette with diluted FA 4. 2. Pipette 25.0  cm 3 of FA 1 into a conical flask. 3. Use the measuring cylinder to add approximately 10.0  cm3 of FA 2 to the same conical flask. 4. Use the measuring cylinder to add approximately 20.0  cm3 of FA 3 to the mixture in the conical flask. The mixture will appear brown, due to iodine produced in the reaction. 5. Begin your rough titration by adding diluted FA 4 from the burette until the intensity of the brown colour decreases. 6. Add 10 drops of starch indicator. The mixture will become darker. 7. Continue titrating until the dark colour is discharged. The mixture should appear off-white. This is the end-point. 8. Add one drop of starch indicator to check that no traces of dark colour are produced. 9. If the mixture stays off-white, the titration is completed. If some dark colour is produced, because iodine is still present, continue the titration until mixture appears off-white. 10. Record your burette readings and the rough titre in the space below.

2 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn Over 11. Carry out as many accurate titrat ions as you think necessary to obtain consistent results. 12. Make sure any recorded results show the precision of your practical work. 13. Record in a suitable form below all of your burette readings and the volume of diluted FA 4 added in each accurate titration. I All the following data is recorded initial and final burette readings for two (or more) titrations II Titre values recorded for accurate titrations, and Appropriate headings and units • initial / start (burette) reading • final / end (burette) reading • titre or volume used / added (not “di