NYJC Prelim 9729 P4 Answers

1 [Type here] H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over NANYANG JUNIOR COLLEGE JC2 PRACTICAL PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME Mark Scheme 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 [Type here] H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over 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 a dilute solution of 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 titration with dilute sodium hydroxide, FB 2. Each titration is to be performed once only, so great care should be taken that you do not overshoot 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 and of the sodium hydroxide. 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 [Type here] 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. Experiment 1 2 3 4 5 Volume of FB 3 added / cm3 5.00 11.00 18.00 24.00 30.00 Initial burette reading / cm3 0.00 20.00 0.00 11.70 21.60 Final burette reading / cm3 15.80 34.00 11.70 21.60 29.80 Volume of FB 2 added / cm3 15.80 14.00 11.70 9.90 8.20 [1]: Correct column or row headings and units for titration recordings Acceptable forms of units: Use of solidus, e.g. / cm 3 Unit in brackets, e.g. (cm3) In words, e.g. volume in cubic centimetres AND All burette readings and/or volumes/titres recorded to 2 dp or to nearest 0.05 cm3) [1]: Follows instructions. Selects three additional volumes of FB 3 which are within the range 5 – 30 cm3 for experiments 2, 3, and 4. AND All gaps between adjacent FB 3 volumes should be ≥ 5 cm3 and ≤ 10 cm3 AND Completes a total of 5 experiments. [2]

4 [Type here] H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over 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 [Type here] H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over 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) = 57.00 cm3…… Vmax(FB 2) = 17.20 cm3…… [5] Teacher’s Vmax(FB 3) diff. Teacher’s Vmax(FB 2) diff. I II III IV V 4 (2.0, 16.6) (55.5, 0.4)

6 [Type here] H2 Chemistry 9729/04 NYJC J2/18 PX [Turn over [1]: All points plotted to within ½ small square in either direction and in the correct square. [1]: The graph line is the straight best-fit line AND Line is correctly extrapolated to intersect both axes. i.e. one continuous straight line AND no point is further than 1 cm3 away from the line in either direction [2]: Accuracy for V max(FB 3) and Vmax(FB 2) [1] Reads correctly, to ±½ small square, the value for Vmax(FB 3) and the value for Vmax(FB 2) If the student has not shown the extrapolation on the graph, the student may still earn this mark if the values quoted correspond to correct extrapolation of the student’s graph line. (ii) Calculate the gradient of your graph line, showing clearly how you did this. gradient = 16.6 0.4 2 55.5 − − = −0.3028 ≈ −0.303 gradient = −0.303………………… [1] To earn this mark, students must • Use clearly