2022 MI H2 Chem Prelim Paper 4 - Answer

Class Adm No Candidate Name: This question paper consists of 19 printed pages and 1 blank page. 2022 Preliminary Examinations Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 31 Aug 2022 2 hours 30 min Candidates answer on the Question paper. READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark 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 at the back of the Question Paper. 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. Question 1 2 3 4 Total Marks 13 17 12 13 55 Shift Laboratory

2 1 Determination of amount of water of crystallization in CuSO4·nH2O Copper is a transition metal capable of exhibiting variable oxidation states. Compounds containing Cu2+ ions tend to be relatively stable. Cu2+ ions react with excess potassium iodide, KI, to produce iodine, I2, and a stable precipitate, CuI. To determine the concentration of Cu2+ via iodometric titration, all the Cu2+ ions are reduced to Cu+ ions. A brown suspension, made up of an off -white precipitate of CuI in a brown solution of I2, will be produced. 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 Na2S2O3 as shown in equation 3. equation 3 I3– + 2S2O32– → S4O62– + 3I– The solution should be titrated immediately after addition of KI because the I2 may be adsorbed onto the CuI precipitate, rendering the end-point less sharp. You are provided with: FA 1 is solid hydrated copper(II) sulfate, CuSO4·nH2O, where n is an integer FA 2 is 0.10 mol dm−3 sodium thiosulfate, Na2S2O3 FA 3 is 1.00 mol dm−3 potassium iodide, KI FA 4 is 10% potassium thiocyanate, KSCN Starch indicator The presence of thiocyanate ion, SCN –, in the titration mixture near to the end -point will have an impact on the accuracy of the results. The procedure described is designed to improve on the accuracy. In this experiment, you will determine the amount of water of crystallisation in 1 mol of CuSO4·nH2O. You will titrate FA 1 against FA 2.

3 [Turn over (a) Procedure Preparation of solution of FA 1 1. Weigh accurately about 5.0 g of FA 1 in a weighing bottle. Record your readings in an appropriate manner in the space provided below. 2. Transfer all the solid into a 250 cm 3 beaker. Dissolve this solid in about 100 cm 3 of deionised water. 3. Transfer the solution to a 250 cm3 volumetric flask. Rinse the beaker with deionised water several times, adding each rinsing to the volumetric flask. 4. Make up the solution to 250 cm3 with deionised water. Stopper, invert and shake well to obtain a homogenous solution. Label this solution as FA 1 solution. Results Mass of empty weighing bottle /g 4.10 Mass of weighing bottle + FA1 /g 9.11 Mass of weighing bottle + residual FA1 /g 4.11 Mass of FA1 /g 5.00 [2] M1: All masses consistently recorded to 2 (or 3) decimal places and correct mass headers M2: Mass of FA 1 used correctly calculated and Mass of FA 1 used 5.00 ±0.05 M1 M2 (b) Titration 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. Titrate FA 1 against FA 2. Near the end-point, when the brown suspension becomes pale yellow, add about 10 drops of starch solution. 5. Continue adding FA 2 until the blue -black colour just disappears. Add 10 cm 3 of FA 4 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. Repeat the titration as many times as you think necessary to obtain consistent results.

4 (i) Results Initial burette reading / cm3 0.00 0.00 Final burette reading / cm3 21.00 21.00 Volume of FA2 added / cm3 21.00 21.00 ✓ ✓ M3: A table including the appropriate header and units for: • Initial burette readings • Final burette readings • volume added M4: All accurate burette readings are recorded to the nearest 0.05 cm3. Do not award this mark if: 50.00cm3 is used as an initial burette reading M5: Accuracy: Titre/mass ratio: 4.05 ± 0.50 [3] M3 M4 M5 (ii) From your titres, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA2 = 21.00+21.00 2 = 21.00 cm3 M6: Correct calculation of the average of two consistent readings within 0.10 cm3 difference Volume of FA 2 = ……………………………. [1] M6 (c) Calculations (i) Calculate the amount of iodine, I2, liberated from 25.0 cm3 of FA 1 solution. amount of S2O32- = 21.00 1000 x 0.10 = 0.00210 mol amount of I2 = ½ x 0.00210 = 0.00105 mol amount of I2 liberated from 25.0 cm3 of FA 1 solution =…….…………………………..[1] M7

5 [Turn over (ii) Hence, calculate the amount of copper(II) ions, Cu2+, in 25.0 cm3 of FA 1 solution. Amount of Cu2+ in 25.0 cm3 = 2 x 0.00105 = 0.00210 mol amount of Cu2+ in 25.0 cm3 of FA 1 solution = ……………………………..[1] M8 (iii) Determine the amount of CuSO4·nH2O in 250 cm3 of FA 1 solution. Amount of Cu2+ in 250 cm3 = 10 x 0.00210 = 0.0210 mol = Amount of CuSO4·nH2O amount of CuSO4·nH2O in 250 cm3 of FA 1 solution = ……………………………..[1] M9 (iv) Calculate the Mr of CuSO4·nH2O, and hence the value of n. [Ar: H, 1.0; O, 16.0; S, 32.1; Cu, 63.5] Mr = 5.00 0.0210 = 238.1 n = 238.1−63.5−32.1−16.0×4 1.0×2+16.0 = 4.36 ≈ 4 (nearest integer) Mr of CuSO4·nH2O = …………………………….. value of n = ……………………… [2] M10 M11 (d) Starch forms a dark blue-black complex with the tri-iodide ion, I3–. The starch indicator is not added at the beginning of the titration as the resulting complex at high I3– concentration is relatively stable, dissociating only slowly. Predict and explain the effect of adding the starch indicator at the start of the titration on the Mr of CuSO4·nH2O determined in (c)(iv). Due to the slow release of I3- from the starch-I3- complex, the end point would have exceeded the equivalence point, resulting in a larger titre value. This leads to larger amount of CuSO4.nH2O hence the Mr would have been smaller than the actual value. [2] M12 M13 [Total: 13]

6 2 Investigation of the effect of concentration changes on the rate of a reaction Iodate(V) ions, IO3−, also react with sulfite ions, SO32−, in the presence of acid to produce iodine: IO3−(aq) + SO32−(aq) + H+(aq) → I2(aq) + other p