Cambridge A Level Chemistry 9701 — 2015 May/June Paper 5 · Variant 2

READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. 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 fl uid. DO NOT WRITE IN ANY BARCODES. Answer all questions. Electronic calculators may be used. You may lose marks if you do not show your working or if you do not use appropriate units. Use of a Data Booklet is unnecessary. 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. CHEMISTRY 9701/52 Paper 5 Planning, Analysis and Evaluation May/June 2015 1 hour 15 minutes Candidates answer on the Question Paper. No Additional Materials are required. Cambridge International Examinations Cambridge International Advanced Level This document consists of 8 printed pages. [Turn over IB15 06_9701_52/FP © UCLES 2015 *3684087800*

2 9701/52/M/J/15 © UCLES 2015 1 This question concerns electrolysis of different compounds. (a) During the electrolysis of dilute sulfuric acid using a current of 0.75 A for 90 minutes, the volume of oxygen gas collected was recorded and is shown in the graph below. 250 200 150 100 50 0 0 10 20 30 40 time / min 50 60 70 80 90 volume of gas / cm3 volume of oxygen (i) Give equations for the reactions that occur at each electrode in the electrolysis of sulfuric acid. … … … … [2] (ii) On the graph above, use a ruler to draw and label a line (hydrogen) to predict the volume of hydrogen that would be given off during the same experiment. [1] (iii) On the graph above, use a ruler to draw and label a line (oxygen) to predict the volume of oxygen that would be produced if a current of 0.45 A was used instead of the 0.75 A used in the original experiment. [1]

3 9701/52/M/J/15 © UCLES 2015 [Turn over (b) During the electrolysis of potassium butanedioate, the following reaction occurs. 2H2O + → C2H4 + 2CO2 + H2 + 2K+OH– CH2COO– K+ CH2COO– K+ potassium butanedioate An experiment can be carried out to confi rm the above equation. In order to do this, the amounts of hydrogen, ethene and carbon dioxide produced need to be measured. Hydrogen is produced at one electrode, ethene and carbon dioxide are produced at the other. The carbon dioxide can be separated from the ethene by absorbing it in an alkali before the volume of ethene is measured. (i) Using the power supply drawn below, draw a fully labelled circuit diagram and apparatus which shows how: ● the current could be measured, ● the hydrogen produced could be collected and its volume measured, ● the carbon dioxide could be removed using a named alkali, ● the volume of ethene could be measured. d.c. power supply + – [5]

4 9701/52/M/J/15 © UCLES 2015 (ii) State what measurements should be taken when carrying out the experiment. … … … … [2] (iii) C coulombs of electricity resulted in V cm3 of hydrogen gas being produced during the electrolysis. In terms of C and V, state the number of coulombs, N, that would be required to produce 24 dm3 of hydrogen. [1] (iv) In terms of N, state the number of faradays of electricity that would be required to produce 1 mol of hydrogen at room temperature and pressure. (1 faraday of electricity = 96 500 coulombs) [1] (v) Give the equation for the reaction that takes place when the carbon dioxide is absorbed by the alkali. Include state symbols. … [1] (vi) Predict the organic product that would be obtained at the anode when a solution of potassium hexanedioate is electrolysed. [1] [Total: 15]

5 9701/52/M/J/15 © UCLES 2015 [Turn over 2 In order to identify a monoprotic (monobasic) hydroxycarboxylic acid, HX, the following experiments are carried out. 25.0 cm3 of an aqueous solution of HX is titrated against 0.0500 mol dm–3 aqueous sodium carbonate. The end-point of the titration is reached when 25.0 cm3 of the aqueous sodium carbonate has been added. (a) (i) Write the equation for the complete neutralisation of HX with sodium carbonate. … [1] (ii) How does the equation show that the concentration of HX is 0.100 mol dm–3? … … [1] (b) (i) State the acid dissociation constant, Ka, for the above reaction in terms of H+ and HX only. Ka = [1] (ii) The pH of the aqueous solution of HX is 2.43. Use the pH and the concentration of HX to show that the pKa of the acid is 3.86. All your working must be shown. [2]

6 9701/52/M/J/15 © UCLES 2015 (c) In an experiment various masses of the sodium salt of the acid, NaX, are added to separate portions of 100 cm3 of HX with stirring. After each addition the pH of the solution obtained is measured. The results of the experiment are recorded in the table below. mass of NaX added / g pH 0.00 2.43 0.10 2.81 0.20 3.11 0.30 3.19 0.40 3.41 0.60 3.59 0.80 3.71 1.00 3.81 1.20 3.89 1.50 3.99 2.00 4.11

7 9701/52/M/J/15 © UCLES 2015 [Turn over (i) Plot a graph to show how the pH of the solution varies with the mass of NaX added. Draw the curve of best fi t. 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 mass of NaX added / g pH 1.6 1.8 2.0 [2] (ii) Circle one anomalous point on your graph and give a reason for how this could have occurred using the experimental procedure described. … … … … [2]

8 9701/52/M/J/15 © UCLES 2015 Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge International Examinations Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cie.org.uk after the live examination series. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge. (d) (i) The graph shows that a pH of 3.86 is obtained when 1.12 g of NaX is added to 100 cm3 of HX. Remember that pKa of HX is also 3.86. Use this information to calculate the relative molecular mass, Mr, of HX. Show your working. [Ar: H, 1.0; C, 12.0; O, 16.0; Na, 23.0] [3] (ii) The calculated Mr is subject to small experimental error. Suggest a structure for the organic hydroxycarboxylic acid, HX, that best fi ts your Mr data. If you have not calculated a value for the Mr, use the value of 104. This is not the correct value. [1] (e) Another method for determining the concentration of the acid HX could be to evaporate a sample of the solution and weigh the solid that remains. Suggest two reasons why this might not be a very good method of fi nding the mass of solid HX in a sample of the solution. … … … … [2] [Total: 15]

® IGCSE is the registered trademark of Cambridge International Examinations. CAMBRIDGE INTERNATIONAL EXAMINATIONS Cambridge International Advanced Level MARK SCHEME for the May/June 2015 series 9701 CHEMISTRY 9701/52 Paper 5 (Planning, Analysis and Evaluation), maximum raw mark 30 This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge will not enter into discussions about these mark schemes. Cambridge is publishing the mark schemes for the May/June 2015 series for most Cambridge IGCSE®, Cambridge International A and AS Level components and some Cambridge O Level components.

Page 2 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 52 © Cambridge International Examinations 2015 Question Expected Answer Mark 1 (a) (i) 2H+(aq) + 2e– ––––> H2(g)
4OH–(aq) ––––> O2(g) + 2H2O(l) +4e– OR 2H2O(l) ––––> O2(g) + 4H+(aq) +4e–
[1] [1] (ii) Any straight line from the origin which has double the oxygen volume at a given time
[1] (iii) Any straight line from the origin which has 0.45 / 0.75 x oxygen volume at a given time
[1] (b) (i) Circuit has an ammeter in series and is complete
Gases are released at the correct electrode
Diagram shows collection of hydrogen using a means of measuring the volume of the gas
Diagram shows carbon dioxide from the anode being absorbed into a named alkali
Diagram then shows ethene being collected using a means of measuring the volume of the gas
[1] [1] [1] [1] [1] (ii) The current / ammeter reading The time taken The volume of hydrogen The volume of ethene Mass of alkali before Mass of alkali after 3 of the above
4 or more of the measurements made
[1] [1]

Page 3 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 52 © Cambridge International Examinations 2015 (iii) (N =) 24 000 x C / V
[1] (iv) N / 96 500
[1] (v) Any correctly balanced equation for the reaction of carbon dioxide and an alkali
[1] (vi) But-2-ene
[1] [15]

Page 4 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 52 © Cambridge International Examinations 2015 2 (a) (i) Na2CO3 + 2HX –––> 2NaX + CO2 + H2O
[1] (ii) 1 mol of Na2CO3 reacts with 2 mol of HX
[1] (b) (i) Ka = [H+]2 / [HX]
[1] (ii) [H+] = 0.00372
[H+]2 / [HX] = 0.000138 OR (answer above)2 / 0.1
OR pKa = 2pH + log[HX]
= 4.86 – 1
[1] [1] (c) (i) All points plotted correctly
Appropriate curve of best-fit is drawn
[1] [1] (ii) Circles the point at mass of NaX = 0.3g
If anomaly is below the line: NaX might not have fully dissolved/mixture not stirred/too little NaX added
If anomaly is above the line; Too much NaX added [1] [1] (d) (i) At pH 3.86, [HX] = [NaX] OR [X–]
Calculates Mr of NaX = 112 or [X–] =89
Calculates Mr of HX as 90
[1] [1] [1] (ii) Structure given has both an –OH and a –COOH group and has rmm = ans(d)(i)
1

Page 5 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 52 © Cambridge International Examinations 2015 (e) Any two from: spitting HX vaporises / evaporates HX decomposes OR is thermally unstable

[2] [15]