2019 EJC Prelim PE H2 Chemistry Paper 4 (solutions comments for students)

© EJC 9729/04/J2PE/19 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2019 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 8 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 03 September 2019 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidental Instructions READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on 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 an HB pencil for any diagrams or graphs. Do not use 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. Qualitative Analysis Notes are printed on pages 19 and 20. 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. Shift Laboratory For Examiner’s Use 1 / 18 2 / 4 3 / 19 4 / 14 Total / 55 This document consists of 18 printed pages.

2 © EJC 9729/04/J2PE/19 For Examiner’s Use Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of water of crystallisation in CuSO4·nH2O Copper forms compounds containing Cu2+ or Cu+ ions. Those compounds containing Cu2+ ions tend to be relatively stable. The addition of an excess of potassium iodide, K I, to a solution of Cu 2+ ions produces iodine, I2, and a stable precipitate of Cu I. To determine the concentration of Cu 2+ via iodometric titration, it is necessary that all the Cu 2+ ions are reduced to Cu + ions. A brown suspension will be produced, which is an off -white precipitate of Cu I in a yellow -brown solution of I2. equation 1 2Cu2+ (aq) + 4I– (aq)  2CuI (s) + I2 (aq) I2 has a relatively low solubility in water. However, the presence of an excess of I– ions in the reaction mixture allows the soluble tri -iodide ion, I3–, to form as shown by equation 2. This ensures that the I2 formed as shown in equation 1 is fully dissolved. equation 2 I2 + I–  I3– The I3– ions formed may be titrated against a standard solution of Na 2S2O3 as shown in equation 3. equation 3 I3– + 2S2O32–  S4O62– + 3I– The solution should be titrated immediately after addition of K I because the I2 may be adsorbed onto the CuI precipitate, rendering the end-point less sharp. FA 1 is solid hydrated copper(II) sulfate, CuSO4·nH2O, where n is an integer. You are also provided with FA 2, 0.100 mol dm–3 sodium thiosulfate, Na2S2O3, FA 3, 1.00 mol dm–3 potassium iodide, KI, 10% potassium thiocyanate, KSCN, 1% starch solution. The presence of thiocyanate ion, SCN –, in the titration mixture near to the end -point will affect the accuracy of the results. The procedure described is designed to improve on the accuracy. In this experiment, you will determine the percentage by mass of water of crystallisation in CuSO4·nH2O. You will titrate a solution of FA 1 against FA 2.

3 © EJC 9729/04/J2PE/19 [Turn Over For Examiner’s Use (a) Preparing a solution of FA 1 1. Weigh accurately about 5.00 g of FA 1 in a pre-weighed weighing bottle. 2. Transfer the solid into a 250 cm3 beaker and reweigh the weighing bottle. Dissolve this solid in about 70 cm3 of deionised water. 3. Transfer the solution to the graduated flask, labelled FA 1 solution. Rinse the beaker with deionised water several times, adding each rinsing to the graduated flask. 4. Make up the solution to 250 cm 3 with deionised water and mix thoroughly. This solution will be used in both Question 1(b) and 2(a). Results (b) Titration of solution of FA 1 against FA 2 (i) 1. Fill the burette with FA 2. 2. Use a pipette to transfer 25.0 cm3 of FA 1 solution into a 250 cm3 conical flask. 3. Use a measuring cylinder to add about 15 cm3 of FA 3 into this flask. 4. Run FA 2 from the burette into this flask. Near the end-point, when the brown solution becomes pale, add about 1 cm3 of 1% starch solution. 5. Continue adding FA 2 until the blue-black colour just disappears. Add 10 cm3 of 10% KSCN solution using a measuring cylinder. 6. Continue adding FA 2 slowly. The end -point is reached when the solution first becomes colourless. The white precipitate remains. 7. Record your titration results in the space provided on page 4. Make certain that your recorded results show the precision of your working. 8. Swirl the reaction mixture and filter the mixture. Wash the residue twice with deionised water. Keep the residue for use in 2(a). The residue is FA 4. Comments:  This part was well done in general. This part was well done in general.  A number omitted to include the mass of empty weighing bottle, as required by the question. Many also did not differentiate between the ‘empty’ and ‘emptied’ bottle in their table. It should be noted that there should not be two same headers within a table.  A small number of students incorrectly referenced mass as “weight”. Mass of weighing bottle and FA 1 / g 9.599 Mass of empty weighing bottle / g 4.596 Mass of emptied weighing bottle / g 4.599 Mass of FA 1 used / g 5.000

4 © EJC 9729/04/J2PE/19 For Examiner’s Use While you are waiting for the mixture to filter, continue with step 9. 9. Repeat points 1 to 7 as necessary until consistent results are obtained. Titration results [5] (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = ................................................. [1] (c) (i) Calculate the amount of iodine, I2, liberated from 25.0 cm3 of FA 1 solution. amount of I2 liberated from 25.0 cm3 of FA 1 solution = .................................................. [1] Comments:  This part was well done in general.  Common errors included using ‘burette volumes’ instead of burette readings, making reference to FA 1 instead of FA 2, and recording volume as ‘amount’.  The accuracy of the titration was poor, with majority of the volumes obtained being significantly higher than the expected volume. Comments:  This part was well done in general. A small number of students incorrectly rounded their values off to 3 sf, although it was left as 2 dp in their final answer. It should be noted that the average volume calculated should follow the precision of the burett e readings, i.e. to 2 dp, without rounding it to 3 significant figures. Titration number 1 2 Final burette reading / cm3 20.00 40.00 Initial burette reading / cm3 0.00 20.00 Volume of FA 2 solution used / cm3 20.00 20.00    Table with correct headings (‘burette’ must be stated) and units.  All burette readings recorded to the nearest 0.05 cm3  Titres for end-point must be within 0.10 cm3   3 20.00 20.00volume of used 2 20.00 cm FA 2 20.00 cm3 2 23 3 SO 20.00 used 0.100 2.00 10 mol1000n               22 3 2 3 3 SO 3 11 2.00 10 mol22 1.00 10 mol n n nI I 1.00  10–3 mol

5 © EJC 9729/04/J2PE/19 [Turn Over For Examiner’s Use (ii) Hence, calculate the amount of copper(II), Cu2+, in 25.0 cm3 of FA 1 solution. amount of Cu2+, in 25.0 cm3 of FA 1 solution = ................................................. [1] (iii) Determine the amount of CuSO4·nH2O in 250 cm3 of FA 1 solution. amount of CuSO4·nH2O in FA 1 solution = ....................................................... [1] (iv) Use your answer to 1(c)(iii) to calculat