NJC Prelim P4_QP

1 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over NATIONAL JUNIOR COLLEGE SH2 Year-End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidates answer on the Question paper Additional Materials: As listed in the Confidential Instructions 9729/04 14 August 2018 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in blue or black pen. You may use an HB 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. Qualitative Analysis Notes are printed on pages 18 and 19. 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. This document consists of 19 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 14 2 / 6 3 / 13 4 / 9 5 /10 Presentation /3 Total / 55

2 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn Over 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