TJC Prelim P4 Ans

1 9729/04/TJC Prelims/2018 [Turn over s CANDIDATE NAME CIVICS GROUP / CENTRE NO. / INDEX NO. / CHEMISTRY 9729/04 Paper 4 Practical 29 August 2018 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your Civics Group and name 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. 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 20 and 21. 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 This document consists of 20 printed pages and 1 blank page. For Examiner’s Use 1 / 24 2 / 14 3 / 17 Total / 55 2018 JC2 PRELIMINARY EXAMINATIONS HIGHER 2

2 9729/04/TJC Prelims/2018 [Turn over BLANK PAGE

3 9729/04/TJC Prelims/2018 [Turn over Answer all the questions in the spaces provided. 1 Determination of the concentrations of sulfuric acid and potassium hydroxide FA 1 is a dilute solution of sulfuric acid, H2SO4 FA 2 is a dilute solution of potassium hydroxide, KOH FA 3 is 0.377 mol dm-3 aqueous sodium carbonate, Na2CO3 You are also provided with thymolphthalein solution. Both potassium hydroxide and sodium carbonate are bases which will react with sulfuric acid to give a salt and water – a process known as neutralisation. You will add different volumes of FA 3 to identical samples of dilute sulfuric acid, FA 1. In each case, the volume of sodium carbonate solution you add will only neutralise part of the sulfuric acid. You will then complete the neutralisation of each mixture by titration with dilute potassium hydroxide, FA 2. Analysis of your results using a graph will enable y ou to determine the concentrations of sulfuric acid and that of potassium hydroxide. (a) (i) Preparation and titration of reaction mixtures Prepare four different mixtures of FA 1 and FA 3 as described below. Mixture 1 1. Pipette 25.0 cm 3 of FA 1 into a conical flask. Label the flask 6.00 cm3. 2. Fill a burette with FA 3. 3. Add 6.00 cm 3 of FA 3 into the same conical flask. 4. Thoroughly stir the mixture and set it aside for later use. Mixture 2 Repeat the procedure used for Mixture 1 but this time add 25.0 cm 3 of FA 1 and 20.00 cm3 of FA 3 into a second conical flask. Label the flask 20.00 cm3. Mixtures 3 and 4 Repeat the procedure used for Mixture 1 but with 9.00 cm3 and 16.00 cm3 of FA 3. In each case, add 25.0 cm 3 of FA 1 and the respective volume of FA 3 into a 100 cm 3 beaker. Thoroughly stir the mixture. Label each 100 cm3 beaker with the volume of FA 3 it contains.

4 9729/04/TJC Prelims/2018 [Turn over Notes: You will perform each titration once only. Great care must be taken that you do not overshoot the end-point. The end-point of the titration of Mixture 1 should be at least 25 cm3 of FA 2. Record your results in the space provided. Make certain that your recorded results show the precision of your working. Titration 1. Fill the second burette with FA 2. 2. Add 3 to 4 drops of thymolphthalein solution to the flask labelled 6.00 cm 3. 3. Titrate the mixture in the flask with FA 2 until a blue colour, which lasts for at least 10 s is obtained. 4. Repeat points 1 to 3 above using your remaining mixtures. You should perform your titrations in order of increasing volume of FA 3. 5. Each of the mixtures stored in beakers should be transferred completely to a third conical flask prior to each titration. Rinse this conical flask thoroughly between each titration. Results Volume of FA 3 / cm3 6.00 9.00 16.00 20.00 Final burette reading / cm3 26.35 47.40 34.85 45.75 Initial burette reading / cm3 0.00 25.00 19.60 35.00 Volume of FA 2 / cm3 26.35 22.40 15.25 10.75 • correct headers and units + 4 sets of reading present and to nearest 0.05 cm3 • correctly calculate all titre vol of FA2 [2] (ii) Plot on the grid below, your values for the FA 2 titre (y-axis) against the volume of FA 3 added (x-axis). You should choose scales for the axes which will allow you to determine by extrapolation • the volume of FA 3 required, V max(FA 3), to react completely with 25.0 cm 3 of FA 1 if no FA 2 is added • the volume of FA 2 required, Vmax(FA 2), to react completely with 25.0 cm 3 of FA 1 if no FA 3 is added Draw the line of best fit, taking into account all of your plotted points. Hence obtain values for V max(FA 3) and Vmax(FA 2).

5 9729/04/TJC Prelims/2018 [Turn over Vmax(FA 3) = …… 29.75 cm3………. Vmax(FA 2) = ……… 32.75 cm3………. Calculate the gradient of your graph line, showing clearly how you did this. Gradient of the graph = 29.75)(0.0 0.00)(32.75 − − = -1.10 gradient = …… -1.10……… [8] • axes correct way round + labels + units + scale • all points plotted correctly based on student table • best-fit straight line + correct extrapolation + no point further than 1 cm 3 away from line in either direction • correctly read both intercept values as Vmax(FA 3) and Vmax(FA 2) based on drawn graph •• accuracy: each titre value within (inclusive) 1.0 cm3 of supervisor • clearly stated coordinates / or in working + at least 3 large squares in each direction • correctly calculated gradient 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 Vol of FA 2 / cm3 Vol of FA 3 / cm3 (0.00, 32.75) (29.75, 0.00)

6 9729/04/TJC Prelims/2018 [Turn over (b) (i) Explain the gradient obtained in (a)(ii) in terms of the chemistry involved. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… [1] (ii) Given that the concentration of aqueous sodium carbonate is 0.377 mol dm -3 and using the appropriate data from your graph, calculate the concentration of sulfuric acid in FA 1. Na2CO3 ≡ H2SO4 No. of moles of Na 2CO3 = 1000 29.750.377× = 0.0112 mol No. of moles of H 2SO4 = 0.0112 mol Concentration of H2SO4 = 25.0 10000.0112 × = 0.448 mol dm -3 concentration of sulfuric acid in FA 1 = …… 0.448 mol dm-3…… [1] (iii) Using your answer to (b)(ii) and appropriate data from your graph, calculate the concentration of potassium hydroxide in FA 2. 2KOH ≡ H2SO4 No. of moles of H 2SO4 in 25.0 cm3 of FA1 added = 0.0112 mol No. of moles of KOH = 0.0112 x 2 = 0.0224 mol Concentration of KOH = 32.75 10000.0224 × = 0.684 mol dm -3 concentration of potassium hydroxide in FA 2 = ...… 0.684 mol dm -3……… [2] • appropriate sf + units in Q1 aii, bii & biii (iv) The concentration of potassium hydroxide in FA 2 can also be calculated using the expression below. [] [] gradient CONa2KOH 32= A student, Jack, performs this experiment and obtains the concentration of potassium hydroxide using the expression, but he calculated his gradient using two points which are close to each other. His classmate, Jill, used the results from Jack’s experiment and calculated the concentration of potassium hydroxide using the method in (b)(ii) and (b)(iii), and insists that her calculated value is more accurat