Cambridge A Level Chemistry 9701 — 2011 May/June Paper 3 · Variant 2

This document consists of 12 printed pages. DC (NF) 33743/4 © UCLES 2011 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Subsidiary Level and Advanced Level * 9 5 7 2 2 5 3 7 0 1 * CHEMISTRY 9701/32 Advanced Practical Skills May/June 2011 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Instructions to Supervisors 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 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 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. Session Laboratory For Examiner’s Use 1 2 3 Total

2 9701/32/M/J/11 © UCLES 2011 For Examiner’s Use 1 Many solid salts exist as hydrates. One example is washing soda – hydrated sodium carbonate, Na2CO3.xH2O. You are to determine the value of x in Na2CO3.xH2O by titration with hydrochloric acid. FB 1 is hydrated sodium carbonate, Na2CO3.xH2O. FB 2 is 0.200 mol dm–3 hydrochloric acid, HCl. methyl orange indicator The equation for the reaction between hydrated sodium carbonate and hydrochloric acid is shown below. Na2CO3.xH2O + 2HCl 2NaCl + CO2 + (x + 1) H2O (a) Method • Weigh the tube containing FB 1, the hydrated sodium carbonate. Record the mass in the space below. • Add all the FB 1 into a 250 cm3 glass beaker. Reweigh the tube containing any residual FB 1. Record the mass in the space below. • Calculate and record the mass of FB 1 used. mass of FB 1 used = … g • Use the 50 cm3 measuring cylinder to add, in total, about 100 cm3 of distilled water to the beaker. • Stir with a glass rod until all the solid has dissolved. • Pour the solution from the beaker into the 250 cm3 graduated (volumetric) flask. • Wash out the beaker thoroughly with distilled water and add the washings to the graduated flask. • Make up the contents of the graduated flask to the 250 cm3 mark with distilled water. • Shake the flask to mix the solution of FB 1. • Pipette 25.0 cm3 of your solution of FB 1 into a conical flask. • Add to the flask a few drops of methyl orange indicator and place the flask on a white tile. • Fill the burette with hydrochloric acid, FB 2. • Titrate the solution of FB 1 with the acid until the end-point is reached.

3 9701/32/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use 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. • Make certain any recorded results show the precision of your practical work. • Record, in an appropriate form below, all your burette readings and the volume of FB 2 added in each accurate titration. [7] (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 1 required … cm3 of FB 2. [1] I II III IV V VI VII

4 9701/32/M/J/11 © UCLES 2011 For Examiner’s Use (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate how many moles of HCl were present in the volume of FB 2 calculated in (b). … mol of HCl (ii) Calculate how many moles of Na2CO3.xH2O were present in 25.0 cm3 of the solution of FB 1. Na2CO3.xH2O + 2HCl 2NaCl + CO2 + (x + 1) H2O … mol of Na2CO3.xH2O (iii) Calculate how many moles of Na2CO3.xH2O were present in 250 cm3 of the solution of FB 1. … mol of Na2CO3.xH2O (iv) Use the mass of FB 1 that you weighed out to calculate the relative formula mass of Na2CO3.xH2O. relative formula mass = … I II III IV V VI

5 9701/32/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use (v) Calculate the value of x in Na2CO3.xH2O. [Ar: H, 1.0; C, 12.0; O, 16.0; Na, 23.0] x = … [6] (d) The error in a single burette reading is ± 0.05 cm3. What is the percentage error in the titre volume calculated in (b)? percentage error = … % [1] [Total: 15]

6 9701/32/M/J/11 © UCLES 2011 For Examiner’s Use 2 You are to determine the enthalpy change for the reaction of hydrated sodium carbonate, Na2CO3.xH2O, with hydrochloric acid, HCl(aq). The equation for this reaction is shown below. Na2CO3.xH2O(s) + 2HCl(aq) 2NaCl(aq) + CO2(g) + (x + 1)H2O(l) FB 3 is hydrated sodium carbonate, Na2CO3.xH2O. FB 4 is 4.00 mol dm–3 hydrochloric acid, HCl. Make sure that in this experiment you use the hydrochloric acid labelled FA 4. (a) Method • Support the plastic cup in a 250 cm3 beaker. • Use a measuring cylinder to transfer 25 cm3 of FB 4 into the plastic cup. • Tilt the beaker so that the bulb of the thermometer is covered by the solution. • Measure and record the temperature of the solution. • Measure and record the mass of the tube containing FB 3. • Carefully tip all the hydrated sodium carbonate from the weighed tube into the plastic cup. • There will be effervescence. Add the solid in small portions with constant stirring using the thermometer. • Record the lowest temperature obtained. • Reweigh the tube containing any residual FB 3. In the space below, record, in an appropriate form, • both balance readings, • both temperature measurements, • the mass of FB 3 used in the experiment, • the fall in temperature. [5] (b) Calculation Show your working and express your answers to three significant figures. (i) Calculate the heat energy change involved in the reaction. (You may assume that 4.3 J are required to change the temperature of 1.0 cm3 of any solution by 1.0 °C.) heat energy produced = … J I II III IV V

7 9701/32/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use (ii) Calculate the number of moles of Na2CO3.xH2O you used in 2(a). You will need to use the relative formula mass you calculated in 1(c)(v). If you were unable to calculate the relative formula mass in 1(c), assume it is 259 but note that this is not the correct value. … mol of Na2CO3.xH2O (iii) Calculate the enthalpy change, in kJ mol–1, for the following reaction. Na2CO3.xH2O(s) + 2HCl(aq) 2NaCl(aq) + CO2(g) + (x + 1)H2O(l) enthalpy change = … … kJ mol–1 sign value [4] (c) In experiments carried out to determine enthalpy changes, heat transfer between the surroundings and the reactants is a significant source of error. This problem can be limited by improved insulation. Apart from modifications made to minimise heat transfer, suggest one possible improvement you could make to the apparatus or procedure to make the determination of the enthalpy change more accurate. … … … [1] [Total: 10] I II III IV

8 9701/32/M/J/11 © UCLES 2011 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 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. Rinse and reuse test-tubes and boiling tubes where possible. Where reagents are selected for use in a test, the full name or correct formula of the reagents must be given. (a) (i) You are provided with three solutions FB 5, FB 6 and FB 7, each of which contains a single cation. One of these cations is aluminium, one is magnesium and the other is lead. Use the information in the Qualitative Analysis Notes on page 11 to select reagents that would enable you to determine the cation in FB 5, FB 6 and FB 7. reagents … Carry out suitable tests and record the results of your experiments in an appropriate form in the space below. [4] I II III IV

9 9701/32/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use (ii) Complete the table below. FB 5 FB 6 FB 7 cation What is the minimum evidence from your observations, that enables you to identify these cations? The minimum evidence for the cation in FB 5 is … … The minimum evidence for the cation in FB 6 is … … The minimum evidence for the cation in FB 7 is … … [4] I II III IV

10 9701/32/M/J/11 © UCLES 2011 For Examiner’s Use (b) You are provided with solid FB 8. Carry out the tests and complete the following table. test observations (i) To a spatula measure of FB 8, in a test-tube, add about a 1 cm depth of distilled water to make a solution. To this solution add 4 pieces of magnesium ribbon. (ii) To a small spatula measure of FB 8, in a boiling tube, add 3 cm depth of aqueous sodium hydroxide. Warm gently and carefully. (iii) To a spatula measure of FB 8, in a test-tube, add about a 1 cm depth of distilled water to make a solution. To this solution add an equal volume of aqueous sodium hydroxide. … … To this mixture add a small volume of hydrogen peroxide. Identify the metal ion present in FB 8. metal ion = … State the change in oxidation number (state) of this metal ion that is occurring in test (i). oxidation number (state) changes from … to … State the change in oxidation number (state) of this metal ion that is occurring in test (iii). oxidation number (state) changes from … to … [7] [Total: 15] I II III IV V VI VII

11 9701/32/M/J/11 © UCLES 2011 [Turn over 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.]

12 9701/32/M/J/11 © UCLES 2011 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. 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) sulfite, 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, Cl2 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) from orange to green

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2011 question paper for the guidance of teachers 9701 CHEMISTRY 9701/32 Paper 32 (Advanced Practical Skills 2), 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. • Cambridge will not enter into discussions or correspondence in connection with these mark schemes. Cambridge is publishing the mark schemes for the May/June 2011 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 AS/A LEVEL – May/June 2011 9701 32 © University of Cambridge International Examinations 2011 Question Sections Indicative material Mark 1 (a) PDO Layout I Volume given for Rough titre and accurate titre details tabulated. Minimum of 2 × 2 boxes. 1 MMO Collection II Initial and final (burette) (readings) and volume of FB 2 added/reading at start and finish recorded for each accurate titre (not ‘difference’). and mass tube + FB 1, mass tube + residue/empty, mass FB 1. Ignore units. Headings should match readings. Do not award this mark 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(.00). 1 PDO Recording III All accurate burette readings (initial and final) recorded to nearest 0.05 (cm3). Assessed on burette readings only (minimum of 2 readings). 1 MMO Decisions IV Has two uncorrected accurate titres within 0.1 cm3. Do not award this mark if, having performed two titres within 0.1 cm3,a further titration is performed that is more than 0.10 cm3 from the closer of the initial two titres, unless a fourth titre, within 0.1 cm3 of any of the previous titres, has also been carried out. 1 Round any burette readings to the nearest 0.05 cm3. Check and correct, if necessary, subtractions in the titre table and in the calculation of mass. Examiner then selects the ‘best’ titre using the hierarchy: two identical; titres within 0.05 cm3, titres within 0.1 cm3 etc. Calculate: candidate’s titre × mass candidate mass Supervisor to 2 decimal places Calculate difference in Supervisor and candidate scaled values and award quality marks as below. MMO Quality V, VI and VII Award V, VI and VII if δ ≤ 0.25 cm3 Award V and VI if 0.25 < δ ≤ 0.50 cm3 Award V if 0.50 < δ ≤ 0.80 cm3 If the ‘best’ titres are ≥ 0.60 cm3 apart cancel one of the Q marks. 3 [7]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 32 © University of Cambridge International Examinations 2011 (b) ACE Interpretation Calculates the mean, correct to 2 decimal places from any accurate titres within 0.2 cm3. The third decimal place may be rounded to the nearest 0.05 cm3. A mean of exactly .×25 or .×75 is allowed but the candidate may round up to .×3 or .×8 or to the nearest 0.05 cm3. If ALL burette readings are given to 1 decimal place then the mean can be given to 1 decimal place if numerically correct without rounding, Mean of 24.3 and 24.4 = 24.35 (✓) Mean of 24.3 and 24.4 = 24.4 () Titres to be used in calculating the mean must be clearly shown – in an expression or ticked in the titration table. Allow ecf from subtraction error for titre. 1 [1] (c) ACE Interpretation I Correctly evaluates step (i) (= mean titre × 0.2 / 1000) II, III and IV are awarded for the correct expression or for the correct answer if no working shown. For all ‘method’ marks, no additional steps can be included. II Step (ii) (answer to (i) / 2) and step (iii) (answer to (ii) × 10) 1 1 PDO Display III In (iv) relative formula mass (= mass of washing soda / answer to (iii)) (ignore g) IV In (v) answer to (iv) – 106 / 18 or 106 + 18x = answer to (iv) (mark method even if Mr is < 106 or very large). V Some relevant working shown in a minimum of four parts in the calculation (in (ii) could be × 2 or ÷ 2, in (iii) could be × 10 or ÷ 10, in (v) could be use of 106). VI In steps (i) to (iv) all answers to 3 or 4 sig figs (minimum of 3 steps). 1 1 1 1 [6] (d) ACE Interpretation 0.1 × 100 / titre from (b) (only expression needed). 1 [1] [Total: 15]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 32 © University of Cambridge International Examinations 2011 2 (a) PDO Layout I Two balance readings, one mass, two thermometer readings and one change in temperature shown in suitable layout. 1 PDO Recording II Masses and temperatures recorded with correct headings and units for all data shown. Acceptable units for temperature are / oC, (oC), temperature in degrees Celsius, temperature in oC., units for mass are /g, (g), mass in grams. 1 PDO Recording III All thermometer readings recorded to 0.0 oC or 0.5 oC and all balance readings recorded to same degree of accuracy. 1 Round all thermometer readings to nearest 0.5 oC.Check and correct, if necessary, subtractions in the temperature change and the mass used. Calculate to 1 decimal place: candidate temperature change × used mass candidate mass Supervisor Calculate difference in candidate and Supervisor scaled values and award quality marks as below. MMO Quality IV and V Award IV and V for changes within 0.8 oC of Supervisor Award V for changes > 0.8 but within 1.6 oC of Supervisor 2 [5] (b) (i) (ii) (iii) ACE Interpretation ACE Conclusions I Expression for heat change in (i) = 25 × 4.3 × temperature change from (a) (answer given must correspond to units quoted). II Expression for moles of washing soda from mass used and Mr from (a) or Mr = 259 or Mr = 286 in (ii) III Correctly evaluates enthalpy change = heat change / (1000 × moles of washing soda) in (iii) (if 1000 not used, must say J). IV Enthalpy change shown as positive and to 3 sig figs. (Answer need not be arithmetically correct). Ignore sig figs (except if approximated to 1 sig fig in rest of question.) 1 1 1 1 [4] (c) ACE Improvements Use a more precise thermometer/a thermometer with more accurate calibrations/a thermometer that reads to 0.1 oC or 0.2 oC (a more accurate thermometer/a digital thermometer/thermocouple is insufficient) or use a more precise method to measure the volume of acid or use a deeper plastic cup or scaling up apparatus and quantities of chemicals used (Do not accept ‘add a lid’) 1 [1] [Total: 10]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 32 © University of Cambridge International Examinations 2011 FB 5 is MgSO4(aq); FB 6 is Pb(NO3)2(aq) FB 7 is Al2(SO4)3(aq); FB 8 is (NH4)2FeSO4(aq) 3 (a) (i) MMO Decisions MMO Collection I Reagents chosen KI(aq) or HCl(aq) or K2CrO4 or K2Cr2O7 or H2SO4 and NaOH (aq) (penalise additional reagents) II NaOH white precipitates for all III Excess NaOH no effect FB 5, precipitate dissolves FB 6 and FB 7 IV KI / HCl / K2CrO4 / K2Cr2O7 / H2SO4 nothing/no visible reaction for (FB 5 and FB 7), yellow precipitate/white precipitate for FB 6. Ignore observations for additional reagents. 1 1 1 1 [4] (ii) ACE Conclusions I FB 5 contains Mg2+, FB 6 contains Pb2+ and FB 7 contains Al3+ (no ecf and must follow observations in (i)) II FB 5 (white) precipitate with NaOH, insoluble in excess III FB 6 (yellow) precipitate with KI / (yellow) precipitate with K2CrO4 or K2Cr2O7 / (white) precipitate with HCl or H2SO4. FB 7 No precipitate with KI / HCl / H2SO4 and (white) precipitate with NaOH, soluble in excess. (Both observations needed unless FB 6 already identified as Pb2+). Allow ecf, based on candidate’s observations, for II, III and IV. 1 1 1 1 [4]

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 32 © University of Cambridge International Examinations 2011 (b) (i) (ii) (iii) MMO Collection Effervescence/bubbles/hydrogen produced (ignore any test for ammonia but tests for other gases negate). (Do not accept gas produced) or Black/grey solid/coating on magnesium Ammonia/gas turns litmus paper blue Green precipitate (any qualified green including grey/green but do not allow green/brown.) Turns brown (any qualified brown) on addition of hydrogen peroxide. Allow rusty or orange/brown precipitate but not orange alone. Ignore effervescence. Fe2+ / iron (II). 1 1 1 1 1 [5] ACE Conclusions (+)2 to 0 (ecf on chromium (+)3 to 0) or (+)3 to (+)2). (+)2 to (+)3. Conclusions are free standing but must be Fe2+. 1 1 [2] [Total: 15]