Cambridge A Level Chemistry 9701 — 2009 May/June Paper 3 · Variant 1

This document consists of 11 printed pages, 1 blank page and 1 Insert. SP (SHW 00157 2/08) T67840/3 © UCLES 2009 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Subsidiary Level and Advanced Level 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 fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. You are advised to show all working in calculations. Use of a Data Booklet is unnecessary. Qualitative Analysis Notes are printed on pages 11 and 12. 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. * 3 1 5 6 8 6 1 6 5 2 * CHEMISTRY 9701/31 Paper 31 Advanced Practical Skills May/June 2009 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions For Examiner’s Use 1 2 3 Total Session Laboratory

2 9701/31/M/J/09 © UCLES 2009 For Examiner’s Use 1 You are provided with the following. FA 1 is 0.15 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is aqueous copper(II) sulfate. You are also provided with a 10% solution of potassium iodide, KI, and starch indicator. You are required to determine the concentration, in g dm–3, of hydrated copper(II) sulfate, CuSO4.5H2O, in FA 2. Dilution of FA 2 (a) By using a burette measure between 47.00 cm3 and 47.50 cm3 of FA 2 into the 250 cm3 graduated flask labelled FA 3. Record your burette readings and the volume of FA 2 added to the flask in the space below. Make up the contents of the flask to the 250 cm3 mark with distilled water. Place the stopper in the flask and mix the contents thoroughly by slowly inverting the flask a number of times. Titration Fill a second burette with FA 1. Perform a rough (trial) titration as follows. Pipette 25.0 cm3 of FA 3 into a conical flask. Use the measuring cylinder provided to add 10 cm3 of 10% potassium iodide to the flask. The Cu2+ ions in FA 3 oxidise the iodide ions to iodine, I2, which can be titrated with FA 1. The flask will also contain an off-white precipitate of copper(I) iodide, CuI. Run FA 1 from the burette, 1 cm3 at a time, until the brown colour of the iodine solution has changed to pale brown. Add approximately 10 drops of starch indicator. A blue-black colour should be seen as the starch reacts with the residual iodine. Continue to add FA 1 1 cm3 at a time until the blue-black colour of the starch-iodine complex disappears and there is no further colour change. In this rough titration … cm3 of FA 1 were added.

3 9701/31/M/J/09 © UCLES 2009 [Turn over For Examiner’s Use Perform sufficient further titrations to obtain reliable results. Record your titration results in the space below. Make certain that your recorded results show the precision of your working. [6] (b) From your titration results obtain a volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. [1] Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (c) Use your answer to (b) to calculate how many moles of Na2S2O3 were run from the burette into the conical flask. ………………… mol of Na2S2O3 were run from the burette into the conical flask. Calculate how many moles of I2 reacted with the Na2S2O3 run from the burette. 2S2O2– 3 S4O2– 6 + 2e– I2 + 2e– 2I– ………………… mol of I2 reacted with the Na2S2O3 run from the burette. Calculate how many moles of Cu2+ ions reacted with iodide ions to produce this amount of I2. 2Cu2+ + 4I– 2CuI + I2 ………………… mol of Cu2+ reacted to form the I2. i ii iii iv v vi

4 9701/31/M/J/09 © UCLES 2009 For Examiner’s Use Calculate the concentration, in mol dm–3, of Cu2+ in FA 3. The concentration of Cu2+ in FA 3 is ………………… mol dm–3. Calculate the concentration, in mol dm–3, of Cu2+ in FA 2. The concentration of Cu2+ in FA 2 is ………………… mol dm–3. Calculate the concentration, in g dm–3, of CuSO4.5H2O in FA 2. [Ar: Cu, 63.5; H, 1.0; O, 16.0; S, 32.1] FA 2 contains ………………… g dm–3 CuSO4.5H2O. [5] (d) The maximum error in any burette reading is ±0.05 cm3. Explain how the maximum error in a titration is therefore ±0.10 cm3. … … … [1] (e) Calculate the maximum percentage error in the average titre given in (b). The error is … %. [1] [Total: 14] i ii iii iv v

6 9701/31/M/J/09 © UCLES 2009 For Examiner’s Use 2 You are to investigate how the rate of formation of sulfur varies with the concentration of sodium thiosulfate, Na2S2O3, in the reaction below. Na2S2O3(aq) + 2HCl(aq) S(s) + 2NaCl(aq) + SO2(g) + H2O(l) Care should be taken to avoid inhalation of SO2(g) that is given off during this reaction. You are provided with the following. FA 1, 0.15 mol dm–3 Na2S2O3 a measuring cylinder to measure 50 cm3 FA 4, 2.0 mol dm–3 HCl a measuring cylinder or marked tube to measure 5 cm3 a printed insert a stop clock or clock with seconds hand (a) Method – Read through the instructions before starting any practical work. • Using the larger measuring cylinder transfer 50 cm3 of FA 1 into a 250 cm3 beaker. • Measure 5 cm3 of FA 4 in the smaller measuring cylinder (or marked tube). • Tip the FA 4 into the FA 1 in the beaker and immediately start timing. • Swirl the beaker to mix the solution and place it on top of the printed insert. • View the printed insert from above so that it is seen through the solution. • Note the time when the printing on the insert just disappears. • Empty and rinse the beaker. Shake out as much of the rinse water as possible and dry the outside of the beaker. • Repeat the experiment using 25 cm3 of FA 1 and 25 cm3 of distilled water. Add 5 cm3 of FA 4 to start the reaction. • Select suitable volumes of FA 1 and distilled water for one further experiment to investigate the effect of sodium thiosulfate concentration on the rate of reaction. Remember to use 5 cm3 of FA 4 and to keep the total volume of FA 1 and distilled water constant. In an appropriate form record the following below: • all measurements of volume and time (to the nearest second) for each experiment, • calculated values of 1/time which are a measure of the rate of reaction. Results [9] i ii iii iv v vi vii viii ix

7 9701/31/M/J/09 © UCLES 2009 [Turn over For Examiner’s Use (b) The total volume in each experiment is constant. Using volumes from the first two experiments, show by simple calculation that the volume of FA 1 used is a measure of its concentration in the reaction mixture. [1] (c) What is the relationship between the rate of reaction and the time taken? … … … [1] (d) For each experiment calculate the numerical value of (volume of FA 1 × time). experiment (volume of FA 1 × time) / (cm3 s) 1 2 3 Use your results in (a) and these calculated values to deduce the relationship between the concentration of Na2S2O3 and the rate of formation of sulfur. … … … [2] (e) Outline briefly how you would modify the experimental method to investigate the effect of temperature change on the reaction rate. … … … [1] [Total: 14]

8 9701/31/M/J/09 © UCLES 2009 For Examiner’s Use 3 FA 5, FA 6, FA 7 and FA 8 are aqueous solutions each containing one cation and one anion. You will carry out specified tests to deduce • the cations present in two of the four solutions, • the anions present in three of the four solutions. 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 identified 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. (a) Carry out the following tests. Record your observations in the spaces provided in the table. FA 5 FA 6 FA 7 FA 8 To 1 cm depth of solution in a test-tube add aqueous sodium hydroxide, a little at a time, until in excess. To 1 cm depth of solution in a test-tube add aqueous ammonia, a little at a time, until in excess. Using the qualitative analysis notes printed on page 11 and the observations above it is possible to identify the cation present in one of the solutions and also to identify possible cations in another of the solutions. Solution … contains the single cation … . Solution … contains one of the following cations, … . [4] i ii iii iv

9 9701/31/M/J/09 © UCLES 2009 [Turn over For Examiner’s Use Rinse and re-use test-tubes. (b) You are to select suitable reagents and carry out tests on the solutions to identify which solution or solutions contain either a nitrate or a nitrite ion. Record in an appropriate form below the tests performed and the observations made. Nitrate or nitrite ions are contained in solution(s) … . [2] (c) Carry out the following tests. FA 5 FA 6 FA 7 FA 8 To 1 cm depth of solution in a test-tube add 1 cm depth of dilute hydrochloric acid. Use these observations to identify the cation or anion present in each solution and complete the table below. solution anion/cation present reason for selecting the ion [4] i ii iii iv i ii

10 9701/31/M/J/09 © UCLES 2009 For Examiner’s Use (d) FA 5 and FA 7 can be mixed to confirm the identity of one ion in each of the two solutions. test observation To 1 cm depth of FA 5 in a test-tube add 1 cm depth of FA 7. This observation confirms the presence of …………… in FA 5 and …………… in FA 7. [2] [Total: 12] i ii

11 9701/31/M/J/09 © UCLES 2009 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, NH+ 4 (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.]

12 9701/31/M/J/09 © UCLES 2009 2 Reactions of anions ion reaction carbonate, CO2– 3 CO2 liberated by dilute acids chromate(VI), CrO2– 4 (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, NO– 3(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO– 2(aq) NH3 liberated on heating with OH–(aq) and Al foil, NO liberated by dilute acids (colourless NO (pale) brown NO2 in air) sulfate, SO2– 4 (aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acid) gives white ppt. with Pb2+(aq) sulfite, SO2– 3 (aq) SO2 liberated with dilute acids; gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acid) 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 acidified aqueous potassium dichromate(VI) (aq) from orange to green 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. University of 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.

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2009 question paper for the guidance of teachers 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 must be read in conjunction with the question papers and the report on the examination. • CIE will not enter into discussions or correspondence in connection with these mark schemes. CIE is publishing the mark schemes for the May/June 2009 question papers for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level syllabuses and some Ordinary Level syllabuses.

Page 2 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – May/June 2009 9701 31 © UCLES 2009 Supervisor’s Report Calculate, correct to 2 dp, the titre if the Supervisor had diluted 47.25 cm3 of FA 2. This is given by the expression titre selected Examiner x diluted volume 47.25 Candidate scripts Calculate the scaled titre for 47.25 cm3 of FA 2. Record the value against the titration table and calculate the difference to Supervisor. Question Sections Indicative material Mark 1 (a) PDO Layout PDO Recording MMO Collection MMO Decisions MMO Quality (i) Tabulates initial and final burette readings and volume added in each of the tables. Do not award this mark if any final and initial burette readings are inverted or 50 is used as the initial burette reading. (ii) Both burette readings in the dilution table and final and initial burette readings for all accurate titres in the titration table recorded to the nearest 0.05 cm3. (iii) Follows instructions: dilutes 47.00 cm3 to 47.50 cm3 and has any two titres within 0.20 cm3 (iv) Has at least two uncorrected “accurate” titres within 0.1 cm3 Do not include any titre labelled “rough”/”trial” unless the candidate has ticked that value or used it in an expression when calculating the average in (b). Accuracy (v) and (vi) Give (v) and (vi) if difference to Supervisor is 0.3 or less Give (vi) only for a difference of 0.3+ to 0.5 Give neither mark for a difference greater than 0.5. 1 1 1 1 2 [6] (b) ACE Interpretation Candidate selects/calculates appropriate “average” from any uncorrected titre values within 0.20 cm3. Candidate is permitted to use a titre labelled “rough” or “trial”. Titres to be used must be shown. Where all titres are given to 1 decimal place the average should be calculated correct to 1 or 2 decimal places. Where any titre is recorded to 2 decimal places, the average should be calculated to 2 decimal places or rounded to the nearest 0.05 cm3. 1 [1]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – May/June 2009 9701 31 © UCLES 2009 (c) ACE Interpretation PDO Display (i), (ii) and (iii) Award three marks if all steps are chemically correct. Withhold 1 mark for each chemical error – no negative marks. Count non-completed steps as chemical errors. step 1 0.15 1000 titre × step 2 × 2 1 step 3 × 2 step 4 × 25 1000 step 5 × diluted volume 250 step 6 × 249.6 (iv) Working shown in at least three of the 5 steps (v) Answers to 3 or 4 significant figures in final answer to each step attempted (minimum of three steps required) 3 1 1 [5] (d) ACE Interpretation Explains that the maximum error is given by + 0.05 cm3 on one burette reading and –0.05 cm3 on the other burette reading, or Individual errors are in opposite directions. 1 [1] (e) ACE Interpretation Calculates titre 0.1 × 100 % Answer must be correct to 2 or 3 decimal places. 1 [1] [Total: 14]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – May/June 2009 9701 31 © UCLES 2009 Supervisor’s Report From the Supervisor’s experimental results round times to the nearest second and calculate the average of (volume of sodium thiosulfate x time) for 50 cm3 and for 25 cm3 of sodium thiosulfate. Candidate’s scripts From the candidate’s experimental results round times to the nearest second and calculate (volume of sodium thiosulfate x time) as above. Record values of (V x t) on script and use in assessing accuracy marks. Question Sections Indicative material Mark 2 (a) PDO Layout PDO Recording MMO Collection MMO Quality (i) Tabulates all experimental readings: volumes of sodium thiosulfate and water, time and rate (1/t) (ii) Single table covering all three experiments A single table has no repetition of column headings. (iii) Table has correct labels and units: e.g. /cm3, /cubic centimetres, or (cm3), (cubic centimetres) or volume in cubic centimetres; Similarly for time (s or seconds but not sec) and rate (s–1, rate (in) per second; 1/s etc.) At least two different units are required. Where units have not been included in the column or row header there should be the appropriate unit for each entry in the table. (iv) All times of reaction are recorded to the nearest second (no decimal places). (v) and (vi) Give (v) and (vi) if difference between candidate’s (V x t) values (50 & 25 cm3 FA 1), is within 5% of the larger value. Give (vi) only if the difference is > 5% but ≤10% of the larger value. 1 1 1 1 2 MMO Decisions (vii) and (viii) Compare the closer of the candidate’s (V x t) values with the Supervisor’s average Vt. Give (vii) and (viii) if difference is within 10% of the Supervisor’s value. Give (viii) only if the difference is > 10% but ≤20% of the Supervisor’s value. (ix) Selects (10–15) or (35–40) cm3 sodium thiosulfate and an appropriate volume of water to give a total volume of 50 cm3 (or 55 cm3 if the volume of acid is tabulated). 2 1 [9]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – May/June 2009 9701 31 © UCLES 2009 (b) ACE Interpretation Candidate shows by calculation or by mathematical expression that [Na2S2O3] ∝ volume of Na2S2O3(aq) in 50/55 cm3 of solution. Reference to (50/50 or 50/55) and (25/50 or 25/55) 1 [1] (c) ACE Conclusions Explains that rate is given by the inverse of time or is inversely proportional to time; or Rate ∝ 1/time Allow Rate = 1/time 1 [1] (d) ACE Interpretation ACE Conclusions (i) Correctly evaluates all Vt values (using times given by candidate, including decimal places where appropriate – or (ii) gives an appropriate qualitative statement relating (rate or time) and concentration. Award this mark if either is correct. Gives a quantitative description of relationship. Vt values are required but do not have to be correctly evaluated. Where no pattern is obvious accept an appropriate statement to that effect. 1 1 [2] (e) ACE Improvements Explains that volumes of reactants or concentration (of thiosulfate and acid) must be kept constant and describes how the temperature will be varied. 1 [1] [Total: 14]

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – May/June 2009 9701 31 © UCLES 2009 Question Sections Indicative material Mark FA 5 is K2CrO4(aq); FA 6 is NaNO2(aq); FA 7 is Pb(NO3)2(aq), FA 8 is MgSO4(aq) 3 (a) MMO Collection ACE Conclusion (i) Records no reaction, no change or no precipitate on adding NaOH and NH3(aq) to FA 5 and FA 6. (ii) Records white ppt soluble (in excess NaOH) and white ppt insoluble (in excess NH3) with FA 7 (iii) Records white ppt insoluble (in excess for both NaOH and NH3) with FA 8 (iv) Conclusion is marked consequentially from the observations for a single cation and a pair of cations. Mg2+/magnesium from white ppt insoluble in an excess of NaOH(aq) and in an excess of NH3(aq) Ca2+/calcium from white ppt insoluble in an excess of NaOH(aq) no ppt in NH3(aq) Pb2+/Al3+ from white ppt soluble in an excess of NaOH(aq) and insoluble in an excess of NH3(aq) Ba2+/NH4 + from no ppt with NaOH(aq) or NH3(aq) FA 6 only 1 1 1 1 [4] (b) MMO Decisions ACE Conclusion (Warms) with NaOH and Al(s) and records appropriate test for ammonia. Gas must be tested in at least one test. This is a mark for the method not the observation. Must have indication that the test has been performed with FA 6, FA 7 and FA 8. In awarding the conclusion mark, assume, in this section only, that a blank box indicates no reaction (no ammonia detected). Award this mark for any of the following: (i) a conclusion, from correct observations, that FA 6 and FA 7 contain nitrate or nitrite (ii) correct observations for NH3 – only with FA 6 and FA 7, but no conclusion given (iii) a statement that NH3 is evolved – only with FA 6 and FA 7 (iv) observation that red litmus turns blue (gas not needed) – only with FA 6 and FA 7 1 1 [2]

Page 7 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – May/June 2009 9701 31 © UCLES 2009 (c) MMO Collection ACE Conclusions (i) Observes a change in colour (from yellow) to yellow/orange or orange (solution), no ppt, with FA 5 and a white ppt with FA 7. (ii) Observes a brown gas formed with only FA 6. Mark (iii) and (iv) consequentially to observations (iii) Give this mark for one conclusion providing it is supported by an acceptable explanation. (iv) Give this mark for two further conclusions supported by acceptable explanations. Minimum acceptable supporting evidence: CrO4 2– from yellow soln or soln turning orange in acid NO2 – from brown gas or from effervescence/fizzing/bubbling with acid, if named soln has yielded ammonia or an alkaline gas in (b) NO3 – no brown gas etc with acid, but ammonia evolved in (b) Pb2+ white ppt with HCl if Pb2+ in (a) (iv) Al3+ no white ppt with HCl if Al3+ in (a) (iv) 1 1 2 [4] (d) MMO Collection ACE Conclusions Mixes FA 5 and FA 7 and observes a yellow ppt. If this section has not been attempted, the correct observation on mixing FA 5 and FA 7 can be carried forward from the conclusions in (c). Concludes that FA 5 contains CrO4 2– and FA 7 contains Pb2+ providing the ions have been previously mentioned in (a) or (c). 1 1 [2] [Total: 12]