NYJC Prelim 9729 P4

NANYANG JUNIOR COLLEGE JC2 PRACTICAL PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS 1 7 REG NO. TUTOR CHEMISTRY 9729/04 Paper 4 Practical 28 August 2018 2 hour 30 minutes Candidates answer on the Question Paper Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name and class 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 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. Shift 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 22 and 23. Laboratory 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. For Examiner’s Use 1 / 28 2 / 11 3 /16 Total /55 This document consists of 23 printed pages and 1 blank page.

2 H2 Chemistry 9729/04 NYJC J2/18 PX Answer all the questions in the spaces provided. 1 To determine the concentrations of aqueous sodium hydroxide and aqueous ethanedioic acid and hence the percentage by mass of sodium ethanedioate in a mixture of sodium ethanedioate and ethanedioic acid. You are provided with the following reagents. • FB 1 is a mixture of approximately 0.01 mol dm −3 aqueous sodium ethanedioate, C2O4Na2, and approximately 0.1 mol dm−3 ethanedioic acid, C2O4H2 • FB 2 is aqueous sodium hydroxide, NaOH • FB 3 is 0.0755 mol dm−3 aqueous sodium carbonate • thymolphthalein indicator Both sodium hydroxide and sodium carbonate are bases which will neutralise ethanedioic acid as shown in reactions 1 and 2. 2NaOH + C2O4H2 → C2O4Na2 + 2H2O reaction 1 Na2CO3 + C2O4H2 → C2O4Na2 + CO2 + H2O reaction 2 In this experiment, you will add different volumes of FB 3 to identical samples of the mixture of aqueous sodium ethanedioate, C 2O4Na2, and ethanedioic acid, C 2O4H2, FB 1. You will then complete the neutralisation of C 2O4H2 in each mixture by titrating with dilute sodium hydroxide, FB 2. Each titration is to be performed once only, so great care should be taken that you do not exceed the end-points. Graphical analysis of your results will enable you to determine the concentrations of the ethanedioic acid in the mixture of aqueous C 2O4Na2, and C2O4H2, FB 1 and of the sodium hydroxide, FB 2. Read through the whole method of conducting the experiment before starting any practical work. The experiment (a) Titrations Experiment 1 1. Fill a burette with FB 3. 2. Pipette 25.0 cm 3 of FB 1 into a 250 cm3 conical flask. 3. Run 5.00 cm 3 of FB 3 into the flask. 4. Add 10 – 15 drops of thymolphthalein indicator to the flask. 5. Fill a second burette with FB 2. 6. Titrate the mixture in the conical flask against FB 2 until the first permanent pale blue colour remains in the solution. 7. Rinse the conical flask thoroughly before it is used for the next experiment. The end-point should be found after the addition of approximately 14 cm 3 of FB 2. One titration, performed accurately, will be sufficient.

3 H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over Experiment 5 1. Repeat Experiment 1 but run 30.00 cm 3 of FB 3 into the flask containing 25.0 cm3 of FB 1. Experiments 2 – 4 Select three other suitable volumes of FB 3 for use in Experiments 2 – 4. Your selected volumes must be between the volumes used in Experiments 1 and 5. Do not use volumes of FB 3 outside the range of 5 – 30 cm3. Repeat the procedures used in Experiment 1 but, in each case, add 25.0 cm3 of FB 1 and your chosen volume of FB 3 into the conical flask. You should perform your titrations in order of increasing volume of FB 3. Rinse this conical flask thoroughly between each titration. Prepare a table in the space below and use it to record the titration results for each volume of FB 3 added. You should record your titration results in order of increasing volume of FB 3 added. [2] I II

4 H2 Chemistry 9729/04 NYJC J2/18 PX 16 14 12 10 8 6 4 2 0 40 30 60 20 50 10 0 18 volume of FB 2 added / cm3 volume of FB 3 added / cm3 (b) (i) Plot, on the grid below, your values for the FB 2 titre (y-axis) against the volume of FB 3 added (x-axis). Draw the line of best fit, taking into account all of your plotted points. Hence obtain values for • the volume of FB 3 required, V max(FB 3), to react completely with 25.0 cm3 of FB 1 if no FB 2 is added; • the volume of FB 2 required, Vmax(FB 2), to react completely with 25.0 cm3 of FB 1 if no FB 3 is added. V max(FB 3) = ……………….. Vmax(FB 2) = ……………….. [5] Teacher’s Vmax(FB 3) diff. Teacher’s Vmax(FB 2) diff. I II III IV V 4

5 H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over (ii) Calculate the gradient of your graph line, showing clearly how you did this. gradient = .....………………… [1] (iii) Explain, in terms of the chemistry involved, the direction of the slope of your graph. …………………………………………………………………………………. …………………………………………………………………………………. …………………………………………………………………………………. ……………………………………………………………………………… [1] Calculations Show your working and appropriate significant figures in all of your calculations. (c) (i) Using appropriate data from your graph, calculate the concentration of ethanedioic acid, C 2O4H2 in FB 1. concentration of C 2O4H2 in FB 1 = ………. ………… [1] (ii) Using your answer to (c)(i) and appropriate data from your graph, calculate the concentration of sodium hydroxide in FB 2. concentration of sodium hydroxide in FB 2 = …………….… [1] (d) Use the expression below to calculate the concentration of sodium hydroxide in FB 2. 232[ ][] || Na CONaOH gradient= concentration of sodium hydroxide in FB 2 = …….…………………… [1]

6 H2 Chemistry 9729/04 NYJC J2/18 PX (e) Student B repeats the experiment described in (a). However, student A had used up FB 3. As such student B has to prepare another 250 cm 3 of FB 3. Unknowingly, he weighs a sample of solid sodium carbonate which is slightly damp for the preparation of FB 3. Suggest and explain what effect this will have on the value of Vmax(FB 3) he obtains. effect on Vmax(FB 3) ………………………………………………………………….. explanation ………………………… ………………………………………………… …………………………………………………………………………………………. …………………………………………………………………………..…………. [1] (f) A student performs the original experiment using solutions of different concentration to those you have used. He determines the gradient of his line using a small triangle. He calculates the concentration of NaOH in FB 2. In part (c)(ii) he obtains a value of 0.589 mol dm−3. In part (d) he obtains a value of 0.581 mol dm−3. Suggest which of these two values is likely to be more accurate. Explain your answer. the more accurate value is ……… ………………………………………………….. explanation …………………… ……………………………………………………… …………………………………………………………………………………………. ……………………………………………………………………………………… [1]

7 H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over (g) Plan an experiment to determine the percentage by mass of sodium ethanedioate, C2O4Na2 in FB 1. Acidified potassium manganate(VII) oxidises ethanedioate ions, C 2O42− ions, as shown below. 2MnO 4−(aq) + 5C2O42−(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l) However, the reaction takes place slowly at first. The rate of the reaction increases a