Cambridge A Level Chemistry 9701 — 2013 May/June Paper 3 · Variant 1

READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Give details of the practical session and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, 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. Qualitative Analysis Notes are printed on pages 12 and 13. 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 9701/31 Advanced Practical Skills 1 May/June 2013 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Confi dential Instructions UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certifi cate of Education Advanced Subsidiary Level and Advanced Level This document consists of 12 printed pages and 4 blank pages. [Turn over IB13 06_9701_31/6RP © UCLES 2013 *0974901356* Session Laboratory For Examiner’s Use 1 2 3 Total

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3 9701/31/M/J/13 © UCLES 2013 [Turn over For Examiner’s Use I II III IV V 1 The reaction between sulfuric acid and sodium hydroxide is exothermic. H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l) By measuring the temperature changes that occur when different volumes of the acid are added to a fi xed volume of the alkali, it is possible to determine the neutralisation point. This is the point at which just enough acid has been added to react with all the alkali present. The aim of the investigation is to determine the concentration of the sulfuric acid. FA 1 is 2.00 mol dm–3 sodium hydroxide, NaOH. FA 2 is dilute sulfuric acid, H2SO4. Read through the instructions carefully and prepare a table for your results before starting any practical work. (a) Method ● Support a plastic cup in a 250 cm3 beaker. ● Use a pipette to transfer 25.0 cm3 of FA 1 into the plastic cup. ● Record the temperature of FA 1, T1, in the space below. T1 = …………. °C ● Fill the burette labelled FA 2 with FA 2. ● Add 5.00 cm3 of FA 2 from the burette to the plastic cup. ● Stir the mixture thoroughly and record the temperature of the solution. ● Add a further 5.00 cm3 of FA 2 to the plastic cup and again record the temperature. ● Repeat the addition of 5.00 cm3 portions of FA 2 until you have added a total of 50.00 cm3 of FA 2 to the plastic cup. Measure the temperature after each addition. ● Record in your table below the total volume of FA 2 added and the temperature of the solution after each addition. [5]

4 9701/31/M/J/13 © UCLES 2013 For Examiner’s Use (b) After each addition of acid, the temperature rise, ∆T, is given by, ∆T = temperature recorded – T1. The total volume of solution in the plastic cup, VT is given by, VT = volume of FA 2 + volume of FA 1. The heat given out by the reaction is proportional to the temperature rise, ∆T, multiplied by the total volume of solution in the plastic cup, VT. Use your experimental results to complete the following table. You should include: ● the volume of FA 2 ● the total volume in the plastic cup, VT ● the temperature of the solution ● the temperature rise, ∆T ● the total volume × the temperature rise, (VT × ∆T) [1]

5 9701/31/M/J/13 © UCLES 2013 [Turn over For Examiner’s Use (c) (i) On the grid below, plot the values of (VT × ∆T) on the y-axis against the volume of FA 2 on the x-axis. I II III IV

6 9701/31/M/J/13 © UCLES 2013 For Examiner’s Use (ii) Draw a straight line of best fi t through the points where the values of (VT × ∆T) are increasing. Draw a second straight line of best fi t through the points where the values of (VT × ∆T) are decreasing. (iii) From your graph, determine the volume of FA 2 where the two lines of best fi t intersect. volume of FA 2 = ……………………………………….. cm3 [5] (d) The value you recorded in (c)(iii) is the volume of FA 2 which is needed to neutralise 25.0 cm3 of FA 1. In the following calculations you will determine the concentration of FA 2. Show your working and appropriate signifi cant fi gures in the fi nal answer to each step of your calculations. (i) Calculate how many moles of sodium hydroxide are contained in 25.0 cm3 of FA 1. moles of NaOH = ………………… mol (ii) Calculate how many moles of sulfuric acid would react with the number of moles of NaOH in (i). moles of H2SO4 = ………………… mol (iii) Calculate the concentration of FA 2. concentration of FA 2 = …………………….. mol dm–3 [3] (e) Other than heat losses from the plastic cup to the surroundings, suggest an additional source of error in this experiment and how this error could be reduced. … … … [1] [Total: 15]

7 9701/31/M/J/13 © UCLES 2013 [Turn over For Examiner’s Use 2 A second way to determine the concentration of an acid is by volumetric titration. In this experiment you will fi rst dilute the sample of FA 2 that you used in Question 1 and then titrate this diluted solution using aqueous sodium hydroxide. H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l) FA 2 is dilute sulfuric acid, H2SO4. FA 3 is 0.150 mol dm–3 sodium hydroxide, NaOH. distilled water (a) Method Dilution of FA 2 ● Use the burette labelled FA 2 to transfer 25.00 cm3 of FA 2 into the 250 cm3 graduated (volumetric) flask, labelled FA 4. ● Make up the contents of the flask to the 250 cm3 mark with distilled water. ● Stopper the flask and mix the contents thoroughly. This is solution FA 4. Titration ● Fill the burette labelled FA 3 with FA 3. ● Use a clean pipette to transfer 25.0 cm3 of FA 4 into a conical flask. ● Add to the flask a few drops of the acid-base indicator provided. ● Titrate the acid in the flask with the alkali, FA 3. You should perform a rough titration. In the space below record your burette readings for this rough titration. The rough titre is …………………… cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration. Make certain that any recorded results show the precision of your practical work. [5] I II III IV V

8 9701/31/M/J/13 © UCLES 2013 For Examiner’s Use (b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value. 25.0 cm3 of FA 4 required …….…….. cm3 of FA 3. [1] (c) (i) Calculate how many moles of NaOH are contained in the volume recorded in (b). moles of NaOH = ………………… mol (ii) Hence, calculate how many moles of H2SO4 are contained in 25.0 cm3 of FA 4. moles of H2SO4 = ………………… mol (iii) Calculate the concentration of the sulfuric acid, FA 2. concentration of FA 2 = …………………….. mol dm–3 [3] (d) You have used two methods to determine the concentration of the sulfuric acid in FA 2. Use your answers to 1(d)(iii) and 2(c)(iii) to calculate the difference in these values as a percentage of the value found by the volumetric titration method. percentage difference = …………………….. % [1] [Total: 10] I II III

9 9701/31/M/J/13 © UCLES 2013 [Turn over For Examiner’s Use 3 Qualitative Analysis At each stage of any test you are to record details of the following. ● colour changes seen ● the formation of any precipitate ● the solubility of such precipitates in an excess of the reagent added Where gases are released they should be identifi ed by a test, described in the appropriate place in your observations. You should indicate clearly at what stage in a test a change occurs. Marks are not given for chemical equations. No additional tests for ions present should be attempted. If any solution is warmed, a boiling tube MUST be used. Rinse and reuse test-tubes and boiling tubes where possible. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. (a) FA 5, FA 6, FA 7 and FA 8 are aqueous solutions each of which contains a single cation and a single anion. Some of the ions present are listed below. Pb2+ Cl – CO3 2– CrO4 2– By observing the reactions that occur when pairs of the solutions are mixed together, you will be able to identify which solution contains which of these ions.

10 9701/31/M/J/13 © UCLES 2013 For Examiner’s Use Use a 1 cm depth of each solution in a test-tube and record your observations in the following table. FA 6 FA 7 FA 8 FA 5 FA 6 FA 7 [8] (b) From your observations deduce which solution contains each of the following ions. ion Pb2+ Cl – CO3 2– CrO4 2– solution [2] (c) Identify another ion that is present in one of the solutions. Explain your reasoning. ion … explanation … … [1] I II III IV V VI VII VIII I II

11 9701/31/M/J/13 © UCLES 2013 [Turn over For Examiner’s Use (d) (i) If chloride ions, Cl –, were to be replaced with bromide ions, Br –, in one of the solutions, would it make any difference to the observations you made in (a)? Explain your answer. … … … (ii) FA 9 is an aqueous solution containing either chloride ions or bromide ions. Select a pair of reagents to identify which anion is present. reagents … Carry out this test and record your observations and conclusion. observations … … The anion in FA 9 is … . [4] [Total: 15]

12 9701/31/M/J/13 © UCLES 2013 Qualitative Analysis Notes Key: [ppt. = precipitate] 1 Reactions of aqueous cations ion reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4 +(aq) no ppt. ammonia produced on heating – barium, Ba2+(aq) no ppt. (if reagents are pure) no ppt. calcium, Ca2+(aq) white ppt. with high [Ca2+(aq)] no ppt. chromium(III), Cr3+(aq) grey-green ppt. soluble in excess giving dark green solution grey-green ppt. insoluble in excess copper(II), Cu2+(aq) pale blue ppt. insoluble in excess blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on contact with air insoluble in excess green ppt. turning brown on contact with air insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess lead(II), Pb2+(aq) white ppt. soluble in excess white ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown on contact with air insoluble in excess off-white ppt. rapidly turning brown on contact with air insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess [Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.]

13 9701/31/M/J/13 © UCLES 2013 2 Reactions of anions ion reaction carbonate, CO3 2– CO2 liberated by dilute acids chromate(VI), CrO4 2–(aq) yellow solution turns orange with H+(aq); gives yellow ppt. with Ba2+(aq); gives bright yellow ppt. with Pb2+(aq) chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)); gives white ppt. with Pb2+(aq) bromide, Br –(aq) gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)); gives white ppt. with Pb2+(aq) iodide, I –(aq) gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)); gives yellow ppt. with Pb2+(aq) nitrate, NO3 –(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2 –(aq) NH3 liberated on heating with OH–(aq) and Al foil; NO liberated by dilute acids (colourless NO → (pale) brown NO2 in air) sulfate, SO4 2–(aq) gives white ppt. with Ba2+(aq) or with Pb2+(aq) (insoluble in excess dilute strong acids) sulfi te, SO3 2–(aq) SO2 liberated with dilute acids; gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2) chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 “pops” with a lighted splint oxygen, O2 relights a glowing splint sulfur dioxide, SO2 turns acidifi ed aqueous potassium dichromate(VI) from orange to green

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CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2013 series 9701 CHEMISTRY 9701/31 Paper 31 (Advanced Practical Skills 1), maximum raw mark 40 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 2013 series for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level components and some Ordinary Level components.

Page 2 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2013 9701 31 © Cambridge International Examinations 2013 Question Sections Indicative material Mark 1 (a) PDO layout I Constructs a table for results with space for 10 volumes. 1 PDO recording II Appropriate headings and units for data given. Volumes in cm3 or / cm3 or (cm3), temperature in °C or /ºC or (ºC) in table. All volumes to same dp. 1 PDO recording III All temperatures recorded to the nearest 0.5 °C both in the table and for T1. At least one ending in .0 and one in .5. 1 MMO quality IV + V Compare temp rise for addition of 25 cm3 of FA 2 with the Supervisor value. Award 2 marks for ∆T within ± 1 °C. Award 1 mark for ∆T within ± 2 °C. 2 [5] (b) ACE interpretation Correctly calculates ∆T, VT and ∆T × VT (assume correct data from (a)) (min 8 results) 1 [1] (c) (i) PDO layout I ∆T × VT on y-axis and volume of FA 2 on x-axis. Axes clearly labelled (ignore units). 1 II Uniform scales chosen to use more than half of each axis. Only include 0 if point plotted. Points plotted use 5 large squares vertically and 4 horizontally. 1 III All points plotted. Examiner to check points at V = 5, 10, 15, 20 and 25. The points should be within ½ small square and in correct small square. Min 8 1 (c) (ii) IV Draws both straight lines of best fit. 1 (c) (iii) ACE interpretation Reads correctly the value of FA 2 from the intercept of the two lines. Answer within 0.5 cm3. Ignore sf. 1 [5] (d) (i) 0.0500 mol (Allow 0.050) 1 (d) (ii) 0.0250 mol (allow 0.025) Allow ecf from (i)/2 1 (d) (iii) 1000 × (d)(ii) / (c)(iii) (2–4 sf) Allow ecf from (ii). Penalise sf once only. 1 [3]

Page 3 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2013 9701 31 © Cambridge International Examinations 2013 (e) ACE improvements Accuracy of temperature measurement – use a 0 to 50 °C thermometer or a thermometer with smaller scale divisions (not just more accurate/ electronic thermometer/parallax). Uncertainty about where the lines cross – sample more values of FA 2 in the region of the intersection. Repeat/ extra readings on LHS of intersection/ near maximum. Initial temperatures of acid and alkali not same – measure both. Other answers acceptable if specific. 1 [1] [Total: 15] 2 (a) MMO collection I Initial and final volumes recorded for rough AND initial, final and volume added recorded for accurate titre. 1 PDO recording II All accurate readings recorded to 0.05 cm3. Do not award if 50(.00) is used as an initial burette reading; more than one final burette reading is 50.(00); any burette reading is greater than 50.(0). 1 MMO decision III Two uncorrected accurate titres within 0.1 cm3. Do not award if, having performed 2 titres within 0.1 cm3, a further titration is performed that is >0.1 cm3 from the closer of the original 2 titres unless a further titration has been carried out which is within 0.1 cm3 of any other. 1 MMO quality IV + V Award 2 marks if difference from Supervisor within 0.20 cm3. Award 1 mark if difference from Supervisor within 0.50 cm3. Examiner compares candidate mean titre with Supervisor mean titre. If best titres are ≥ 0.5 cm3, cancel one of the Q marks. 2 [5]

Page 4 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2013 9701 31 © Cambridge International Examinations 2013 (b) ACE interpretation Calculates the mean to appropriate decimal places. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Example: 26.667 must be rounded to 26.67. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp and the mean is exactly correct. eg 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect. Note: the candidate’s mean will sometimes be marked as correct even if it is different from the mean calculated by the Examiner for the purpose of assessing accuracy. 1 [1] (c) ACE interpretation All answers correct. (i) 0.15 × (b) /1000 (ii) (i)/2 (iii) (ii) × 400 1 PDO display Working shown in (i) and (iii) 1 PDO display All answers given to 3 or 4 sig figs (minimum 2). 1 [3] (d) ACE interpretation Correctly works out % difference to min 2 sig figs. 1 [1] [Total: 10]

Page 5 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2013 9701 31 © Cambridge International Examinations 2013 FA 5 = HCl FA 6 = Pb(NO3)2 FA 7 = K2CrO4 FA 8 = Na2CO3 FA 9 = KBr 3 (a) MMO collection I FA 5 and FA 6: a white ppt insoluble in excess 1 II FA 5 and FA 7: solution turns from yellow or colourless to orange 1 III FA 5 and FA 8: bubbles or effervescence 1 MMO decisions IV Uses limewater to test for gas and result. 1 ACE conclusion V Identifies gas as CO2. 1 MMO collection VI FA 6 and FA 7: a yellow ppt (insol in excess) 1 VII FA 6 and FA 8: a white ppt (insol in excess) 1 VIII FA 7 and FA 8: no reaction/ colourless to yellow (solution) 1 [8] (b) ACE conclusion Pb2+ Cl – CO3 2– CrO4 2– FA 6 FA 5 FA 8 FA 7 4 correct scores 2 marks 3 correct scores 1 mark 2 [2] (c) ACE conclusion H+ because of colour change with chromate or CO2 / gas released with carbonate. 1 [1] (d) (i) ACE conclusion No as PbBr2 / lead bromide is also a white ppt / gives the same observation (if correct in table). 1 (d) (ii) MMO decision Add AgNO3 followed by NH3. 1 MMO collection Cream ppt partially soluble or insoluble in ammonia/ soluble in conc. NH3. 1 ACE conclusion Bromide / Br can be allowed from ‘off white / buff’. 1 [4] [Total: 15]