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 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. 11. Carry out as many accurate titrat ions as you think necessary to obtain consistent results.

3 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over 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. [7] I II III IV V VI VII From your accurate titration results, obtain a suitable value for the volume of diluted FA 4 to be used in your calculations. Show clearly how you obtained this value. The iodine produced required ............................. cm 3 of diluted FA 4. [1] (b) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium thiosulfate, Na 2S2O3, in the volume of diluted FA 4 obtained in (a). moles of Na 2S2O3 = ............................. mol [1]

4 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn Over (ii) Balance the ionic equation for the reaction of iodine with sodium thiosulfate. State symbols are not required. …..... I2 + ........ S2O32– → ........ S4O62– + ……... I– Hence calculate the number of moles of iodine that reacted with the number of moles of Na 2S2O3 calculated in (i). moles of I2 = ............................. mol [1] (iii) Using your answer to (ii), calculate the number of moles of copper( II) ions in 25.0 cm3 of FA 1. moles of Cu 2+ ions = ............................. mol [1] (iv) Using your answer to (iii) and the information on page 2, calculate the average relative formula mass of the copper salts in FA 1. Average Mr of copper salts = ............................. [1] (v) Write the full electronic configuration of 29Cu in CuI.

5 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over ………………………………………………………………………..…………[1] (vi) Hence, explain why solid Cu I appears white in colour. ………………………………………………………………………..…………..… ………………………………………………………………………..……….….… ………………………………………………………………………..………..…… [1] [Total: 14]

6 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn Over 2 Qualitative analysis In this question, you will deduce the two anions present in FA 1. Perform the tests described in Table 2 and record your observations in the table. Test and identify any gases evolved. If any solution is warmed, a boiling tube MUST be used. Table 2 Test Observations (a) To a 1 cm depth of FA 1 in a test- tube, add aqueous silver nitrate. (b) To a 0.5 cm depth of FA 1 in a boiling tube, add aqueous sodium hydroxide and add one piece of aluminium foil and warm. (c) To a 1 cm depth of FA 1 in a test- tube, add aqueous barium chloride followed by nitric acid. (d) To a 1 cm depth of FA 1 in a test- tube, add an equal volume of sulfuric acid followed by KMnO 4. [3] (e) From your observations, state the anions present in FA 1. Explain your answers. First anion: ………………………… Second anion:………………………… ………………………………………………………………………………………………..… ………………………………………………………………………………………………..… ………………………………………………………………………………………………..… ………………………………………………………………………………………………..… ………………………………………………………………………………………………….. [3] [Total: 6]

7 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over 3 Investigation of thermal decomposition of sodium hydrogencarbonate Sodium hydrogencarbonate, NaHCO3, is used as baking soda in cooking. Baking soda may also contain small amounts of other chemicals. When baking soda is heated, carbon dioxide is produced. In this experiment, you will investigate the reaction taking place when the sodium hydrogencarbonate in baking soda is thermally decomposed. FA 5 is baking soda (impure NaHCO 3). (a) Method Record all your readings in the space below. 1. Weigh the crucible with its lid. 2. Transfer all the FA 5 from the container into the crucible. 3. Weigh