SAJC 2022 P4 ANS

1 Name: Shift: Class: 21S Lab: ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 18 Aug 2022 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes For Examiner’s Use 1 2 3 4 Total This document consists of 22 printed pages, including 1 blank page. READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, 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. The number of marks is given in the brackets [ ] at the end of each question or part question. 19 14 14 8 55

2 1 Determination of water of crystallisation in a sample of hydrated copper(II) sulfate The formula of hydrated copper( II) sulfate is CuSO 4•xH2O, where x refers to the number of moles of water of crystallisation. In 1(a)(i), you will perform titration to determine the value of x. Excess aqueous KI is first added to copper(II) sulfate solution. This will produce a white precipitate in a brown solution of I2. equation 1 Cu2+ (aq) + 2I–(aq) → CuI (s) + ½I2 (aq) The liberated iodine is then titrated against sodium thiosulfate. equation 2 I2 (aq) + 2S2O32– (aq) → 2I– (aq) + S4O62– (aq) You are provided with: • solid FA 1, hydrated copper(II) sulfate • FA 2, 0.100 mol dmꟷ3 sodium thiosulfate, Na2S2O3 • FA 3, potassium iodide solution, KI • Solution S, starch solution. Note: Solution S will also be used in Question 2. (a) (i) Procedure 1. Fill a burette with FA 2. 2. Weigh accurately about 5 g of FA 1. Record the mass on page 3. Transfer all the solids into a 100 cm 3 beaker. Add about 75 cm3 of deionised water and stir with a glass rod to dissolve them. 3. Transfer the solution into a 250 cm 3 volumetric flask. Rinse the beaker with deionised water and pour the washings into the volumetric flask. 4. Make up to the 250 cm3 mark with deionised water. Stopper the volumetric flask and shake well to mix. Label this solution as FA 4. 5. Pipette 25.0 cm3 of FA 4 into a conical flask and use a measuring cylinder to add 15 cm3 of FA 3 into the same conical flask. 6. Titrate this solution with FA 2 until the mixture becomes pale brown. An off-white precipitate is also present in the conical flask.

3 7. Add approximately 1 cm3 of solution S to the conical flask and continue titration until the blue-black colour just disappears, with the off-white precipitate remaining in the conical flask. 8. Discard the contents and rinse the conical flask with water. 9. Repeat the titration to obtain consistent results. Record your titration results in the space below. Results Mass of empty weighing bottle / g Mass of FA 1 and weighing bottle / g Mass of emptied weighing bottle / g OPTIONAL Mass of FA 1 / g 5.03 / 5.00 / 5.03 1 2 3 final burette reading / cm3 initial burette reading / cm3 volume of FA 2 used / cm3 20.20 20.00 19.95 values used for calculating average titre • Mass of FA 1 used should be 5 +/- 0.05 g [1] • Tabulates correct headers and units for both mass and titration [1] • All values are recorded to 2 decimal places and to nearest 0.05 cm3 [1] • Two titre volumes within 0.10cm3 [1] [4]

4 (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. • Average of 2 consistent titre volumes, recorded to 2 d.p [1] Volume of FA 2 = ………………………. cm3 [1] (b) (i) Calculate the amount, in moles, of Cu2+ present in 25.0 cm3 of FA 4. Amount of S2O32ꟷ = (average titre volume / 1000) x 0.1 = Amount of Cu2+ in 25 cm3 of FA 4 [1] ecf from (a)(ii) Amount of Cu2+ = ………………………. mol [1] (ii) Calculate the concentration of Cu2+ in FA 4. Concentration of Cu2+ in FA 4 = Ans from (b)(i) / 0.025 [1] ecf from (b)(i) Concentration of Cu2+ = ……………………… mol dm−3 [1] (iii) Given that the concentration of CuSO4•xH2O in FA 4 is 20.00 g dm−3, use your answer in (b)(ii) to determine the value of x. [Ar: H, 1.0 Cu, 63.5 S, 32.1 O, 16.0] Molar mass of hydrated CuSO4 = 20.00 / ans from (b)(ii) = _______ [1] g mol-1 Molar mass = (63.5 + 32.1 + 64 + x18) x ≈ 5 [1; rounded off to nearest whole number] [1] for unit and 3 sf for (b)(i) and (b)(ii) Accuracy mark [1] for x ≈ 5 (from excel calculator) x = ……………………….. [4]

5 (iv) In step 5, FA 3 was added using a 25 cm 3 measuring cylinder . Calculate the percentage error in the measurement of the specified volume in step 5. % error =  (0.25 / 15) x 100% =  1.67% [1] Annotate if student did not calculate to 3 sf and annotate  Percentage error = ……………………….. % [1] (c) (i) A student conducted the experiment as mentioned in (a)(i). After carrying out step 5, he left the conical flask containing FA 3 and FA 4 to stand for 30 minutes, before continuing with steps 6 to 8. It was noted that his titre volume was less than expected. Suggest a reason for this observation. ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… [1] Iodine is volatile and some will escape from the conical flask. [1] Or words to the effect, e.g. evaporation of iodine. (ii) Two other students conducted the same experiment in (a)(i) but with the following modifications to the procedures. Student 2: In Step 5, add 30 cm3 of FA 3 instead of 15 cm3. Student 3: In Step 7, omit the use of solution S. Explain how each of these modifications will affect the accuracy of the results. Student 2: Potassium iodide was already added in excess in (a)(i) / The amount of iodine formed will be the same for both 15 cm 3 and 30 cm 3 of FA 3. / Cu2+ is limiting reagent. Hence, it will not be affected the accuracy of the results / the results will be the same. [1] Student 3: The colour change at end-point from yellow / pale brown to colourless will not be sharp / obvious, thus leading to inaccuracy in the titre volumes recorded. [1]

6 (iii) Another student proposed the following modification. • Filtering the contents in the conical flask after step 5, before carrying out the titration in step 6. State an advantage and a disadvantage of this modification. Advantage: Easier to see the colour change at end-point without the interference of white ppt. [1] Disadvantage: Some iodine may be lost OR remain on the filter paper , thus reducing the accuracy of titration results. [1] (d) Table 1.1 shows some standard electrode potential values. Table 1.1 electrode reaction Eo / V Cu2+ + e– ⇌ Cu+ +0.15 I2 + 2e– ⇌ 2I− +0.54 The Eocell for equation 1 is −0.39 V. Explain why the reaction occurred in (a)(i) despite the negative Eocell value. When CuI ppt is formed, [Cu+] / amount of Cu 2+ decreases. By Le Chatelier’s Principle, the position of equilibrium for Cu2+ + e– ⇌ Cu+ shifts right [1] to form more Cu+, resulting in its Ecathode (or words to the effect) to be more positive and hence, Ecell will become positive. [1] [Total: 19]

7 2 Determination of the kinetics of the iodide-peroxodisulfate redox reaction This question seek