VJC Prelim P4 QP and Ans

© VJC 2018 9729/Prelim/18 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/04 Practical 30 August 2018 2 hours 30 minutes Additional Materials: As listed in the instructions below READ THESE INSTRUCTIONS FIRST Write your name and CT group on all 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 a 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 14 and 15. Periodic Table is printed on page 16. 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 2 3 4 Total This document consists of 16 printed pages.

2 © VJC 2018 9729/04/Prelim/18 1 You are to determine the concentration, in g dm −3, of sodium ethanedioate in a mixture of sodium ethanedioate and ethanedioic acid. This experiment involves two titrations. In titration one, you will carry out a titration to find the total amount of ethanedioate ion, C 2O42−. In titration two, you will use the information provided to find the amount of ethanedioic acid, H 2C2O4. Finally, you will use the values found in the two titrations to calculate the concentration, in g dm −3, of sodium ethanedioate in FA 1. FA 1 is a mixture of aqueous sodium ethanedioate, Na 2C2O4, and ethanedioic acid, H2C2O4. FA 2 is approximately 2 mol dm−3 sulfuric acid, H2SO4. FA 3 is 0.0200 mol dm−3 potassium manganate(VII), KMnO4. Titration One (a) 1. By using a burette, measure between 42.50 cm 3 of FA 1 into the 250 cm 3 graduated (volumetric) flask. 2. Record your burette readings and the volume of FA 1 added to the flask in the space below. Final burette reading / cm3 42.50 Initial burette reading / cm3 0.00 Volume of FA 1 used / cm3 42.50 [1] 3. Make up the contents of the flask to the 250 cm 3 mark with deionised water. Place the stopper in the flask and mix the contents thoroughly by slowly inverting the flask a number of times. Label this solution FA 4. 4. Fill a second burette with FA 3. 5. Pipette 25.0 cm 3 of FA 4 from the graduated flask into a conical flask. 6. Use a measuring cylinder to add 25 cm 3 of FA 2 to the conical flask. 7. Place the conical flask on a tripod and gauze and heat to about 65 oC. 8. If the neck of the flask is too hot to hold safely, use a folded paper towel to hold the flask. 9. Titrate the mixture in the conical flask with FA 3 until a permanent pale pink colour is obtained. This is the end-point. 10. If a brown colour appears during the titration, reheat the flask to 65 oC. The brown colour should disappear and the titration can be completed as above. If the brown colour does not disappear on reheating , discard the solution and start the titration again. 11. Carry out as many titrations as you think necessary to obtain consistent results. 12. Record in an appropriate form all of your burette readings and the volume of FA 3 added in each titration. Final burette reading / cm 3 19.50 39.50 Initial burette reading / cm3 0.00 20.00 Volume of FA 3 used / cm3 19.50 19.50 [5]

3 © VJC 2018 9729/04/Prelim/18 (b) From your titration results, obtain a suitable volume of FA 3 to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 3 used = (19.50 + 19.50) ÷ 2 = 19.50 cm 3 25.0 cm3 of FA 4 required ………………… cm 3 of FA 3. [1] Titration Two (c) When 25.0 cm 3 of FA 4 used in (a) is titrated with 0.100 mol dm −3 sodium hydroxide using phenolphthalein as the indicator, 15.50 cm 3 of sodium hydroxide is needed for complete reaction. (i) Write an equation for the reaction between sodium hydroxide and ethanedioic acid. ………………………………..…………………………………………………………….… [1] (ii) Calculate the number of moles of sodium hydroxide required to react with 25.0 cm 3 of FA 4. No. of moles of NaOH required = 0.100 × 15.50 × 10 –3 = 1.55 × 10–3 mol moles of NaOH = ………………… mol [1] (iii) Hence, calculate the number of moles of ethanedioic acid in 25.0 cm 3 of FA 4. No. of moles of H 2C2O4 present in 25.0 cm3 = ½ × no. of moles of NaOH required = ½ × 1.55 × 10–3 = 7.75 × 10–4 mol moles of H2C2O4 in 25.0 cm3 of FA 4 = ………………… mol [1] (d) (i) Use your answer from (b) to calculate the number of moles of potassium manganate(VII), FA 3, required to react with 25.0 cm3 of FA 4 in Titration One. No. of moles of KMnO 4 required = 0.0200 × 19.50 × 10–3 = 3.90 × 10–4 mol moles of KMnO 4 = ………………… mol [1] (ii) The equation for the reaction between acidified manganate(VII) ions and ethanedioate ions is shown below. 2MnO 4−(aq) + 5C2O42−(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l) Use your answer from (d)(i) to calculate the total number of moles of ethanedioate ions in 25.0 cm 3 of FA 4. Total no. of moles of C 2O42− present in 25.0 cm3 = 5/2 × no. of moles of MnO4− required = 5/2 × 3.90 × 10–4 = 9.75 × 10–4 mol total moles of C2O42− in 25.0 cm3 of FA 4 = ………………… mol [1] 19.50 H2C2O4 + 2NaOH → Na2C2O4 + 2H2O 1.55 × 10–3 7.75 × 10–4 3.90 × 10–4 9.75 × 10–4

4 © VJC 2018 9729/04/Prelim/18 (iii) Use your answers from (c)(iii) and (d)(ii) to calculate the number of moles of ethanedioate ions which came from the sodium ethanedioate in 25.0 cm3 of FA 4. No. of moles of C 2O42− from Na2C2O4 in 25.0 cm3 = (9.75 × 10–4) − (7.75 × 10–4) = 2.00 × 10–4 mol moles of C2O42− from Na2C2O4 in 25.0 cm3 of FA 4 = ………………… mol [1] (iv) Hence, calculate the concentration, in g dm −3, of sodium ethanedioate in FA 1. No. of moles of Na 2C2O4 in 250 cm3 of solution of FA 4 = 2.00 × 10–4 × 250 25.0 = 2.00 × 10–3 mol Mass of Na 2C2O4 in 42.70 cm3 of FA 1 = 2.00 × 10–3 × (2×23.0 + 2×12.0 + 4×16.0) = 0.268 g Concentration, in g dm −3, of Na2C2O4 in FA 1 = 0.268 ÷ 42.50 1000 = 6.31 g dm −3 Concentration of sodium ethanedioate is ………………… g dm −3 [2] (e) Explain why the decolourisation of FA 3 in Titration One is initially slow but becomes faster as the reaction proceeds. …………………………………………………..……………………………………….…………... …………………………………………………..……………………………………….…………... …………………………………………………..……………………………………………..….. [1] [Total: 16] Sufficient auto-catalyst, Mn 2+(aq) ions, will be formed to catalyse the reaction to occur faster. 2.00 × 10–4 6.31

5 © VJC 2018 9729/04/Prelim/18 2 This question concerns the solubility of FA 5, potassium bromate(V), KBrO3, in water. The solubility of a substance in water is defined as: the mass of substance that will dissolve in and just saturate 100 g