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

This document consists of 12 printed pages. DC (NH) 33739/4 © UCLES 2011 [Turn over * 7 2 6 4 0 6 7 8 7 5 * 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 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. CHEMISTRY 9701/33 Advanced Practical Skills May/June 2011 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Instructions to Supervisors For Examiner’s Use 1 2 3 Total Session Laboratory

2 9701/33/M/J/11 © UCLES 2011 For Examiner’s Use 1 FA 1 is an iron salt in which all the iron is present as Fe2+ cations. You will work out the percentage of iron in this salt by titrating a solution of this salt with a standard solution aqueous potassium manganate(VII). FA 1 is an unknown iron(II) salt. FA 2 is 1.00 mol dm–3 sulfuric acid. FA 3 is 0.0100 mol dm–3 potassium manganate(VII). (a) Method Weighing out the salt • Weigh the tube containing FA 1. • Tip the contents of the tube into a 250 cm3 beaker. • Re-weigh the empty tube. • Record all your readings in a suitable form in the space below. Preparing the solution • To the salt in the beaker use a measuring cylinder to add approximately 200 cm3 of FA 2 and stir until the salt has dissolved. • Pour the contents of the beaker carefully into the 250 cm3 graduated (volumetric) flask using the small funnel. • Rinse the contents of the beaker twice with a little distilled water and add these washings to the graduated flask. • Fill the graduated flask to the line with distilled water. Shake carefully to ensure adequate mixing. Titration • Fill the burette with FA 3. • Pipette 25.0 cm3 of the solution of FA 1 from the graduated flask into a conical flask. • Titrate the solution of FA 1 in the flask with FA 3 until the first appearance of a permanent pink colour. You should perform a rough titration. In the space below record your burette readings for this rough titration. The rough titre is … cm3.

3 9701/33/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use • Carry out as many accurate titrations as you think are necessary to obtain consistent results. • Make certain any recorded results show the precision of your practical work. • Record in an appropriate form below all of your burette readings and the volume of FA 3 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 have obtained this value. 25.0 cm3 of the solution of FA 1 required … cm3 of FA 3. [2] I II III IV V VI VII

4 9701/33/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 MnO4 –(aq) were present in the volume of FA 3 calculated in (b). moles of MnO4 –(aq) = … mol (ii) Use the following equation to calculate how many moles of Fe2+(aq) were present in the conical flask. MnO4 –(aq) + 8H+(aq) + 5Fe2+(aq) Mn2+(aq) + 5Fe3+(aq) + 8H2O(aq) moles of Fe2+(aq) in the conical flask = … mol (iii) Calculate the number of moles of Fe2+ in your weighed sample of FA 1. moles of Fe2+ in the weighed sample = … mol I II III IV V

5 9701/33/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use (iv) Calculate the percentage of iron in FA 1. [Ar: Fe, 55.8] the percentage of iron in FA 1 = … % [5] (d) There are a number of sources of potential error in this experiment. One of these involves the readings taken using the balance. (i) State the maximum individual error in any single balance reading. maximum individual error = … g (ii) Calculate the maximum percentage error in the mass of FA 1 used in your experiment. maximum percentage error = … % [2] [Total: 16]

6 9701/33/M/J/11 © UCLES 2011 For Examiner’s Use 2 FA 4 is an impure sample of hydrated magnesium sulfate, MgSO4.7H2O. When heated the water of crystallisation is driven off to leave anhydrous magnesium sulfate, MgSO4. The impurity does not give off water when heated. By determining how much water is present in the impure sample, the percentage purity can be calculated. (a) Method • Weigh a clean dry crucible. • Empty all of the FA 4 into the crucible. • Reweigh the crucible and its contents. • Support the crucible in the pipe-clay triangle on top of a tripod. • Heat the crucible gently for about 1 minute and then more strongly for a further 4 minutes. • Allow the crucible to cool. You should start question 3 while cooling is taking place. • When the crucible is cool enough to handle, reweigh the crucible and its contents. • Repeat the cycle of heating and weighing as many times as you think necessary. In the space below, record, in an appropriate form, all your weighings and include the mass of FA 4 used and the mass of water that was lost. [5] I II III IV V

7 9701/33/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use (b) Calculations Show your working and express your answers to three significant figures. (i) Using the mass of water that was lost on heating, calculate the mass of MgSO4.7H2O that was present in the initial sample of FA 4. [Ar: H, 1.0; O, 16.0; Mg, 24.3; S, 32.1] mass of MgSO4.7H2O = … g [1] (ii) Calculate the percentage by mass of MgSO4.7H2O in FA 4. percentage by mass of MgSO4.7H2O in FA 4 = … % [1] (c) Suggest an improvement to the practical procedure that would give a more accurate value for the percentage by mass of MgSO4.7H2O in FA 4. … … … [1] [Total: 8]

8 9701/33/M/J/11 © UCLES 2011 For Examiner’s Use Qualitative Analysis 3 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. FA 5 is a sodium salt. FA 6 is a salt containing a single cation and a single anion from those listed in the Qualitative Analysis Notes on pages 11 and 12. FA 7 is an aqueous solution of an unknown compound. By carrying out specific tests you will identify some of the ions in these. (a) Put a spatula measure of FA 5 into a boiling tube. Heat it gently for a few minutes and then strongly until no further changes are seen. Record your observations at each stage, in the space below. [2] Leave the boiling tube to cool. Do not discard the contents as they will be used later in the question. (b) Put a spatula measure of FA 6 into a test-tube. Half fill the test-tube with distilled water and dissolve the solid. You will use this solution to carry out the following tests. • To a 1 cm depth of a solution of FA 6 in a boiling tube, add 0.5 cm depth of aqueous sodium hydroxide using a teat pipette. Heat the mixture carefully. • To a 1 cm depth of a solution of FA 6 in a test-tube, add aqueous ammonia. • To a 1 cm depth of a solution of FA 6 in a test-tube, add aqueous barium chloride or barium nitrate. • To a 1 cm depth of a solution of FA 6 in a test-tube, add aqueous silver nitrate, followed by aqueous ammonia.

9 9701/33/M/J/11 © UCLES 2011 [Turn over For Examiner’s Use Record your observations for each of the tests in the space below. Identify the ions present in FA 6. FA 6 contains … and … [6] (c) Put a small spatula measure of FA 6 into a boiling tube. Taking great care, add 5 drops of concentrated sulfuric acid. CARE: Concentrated sulfuric acid is very corrosive. Once you have made your observations fill the boiling tube with water. Record your observations in the space below. What type of chemical reaction occurs between FA 6 and sulfuric acid? Justify your answer. … … [3]

10 9701/33/M/J/11 © UCLES 2011 For Examiner’s Use (d) To the residue in the boiling tube from (a), slowly and carefully add FA 7 to a depth of about 5 cm. Divide this solution equally into two test-tubes. • To one test-tube add 5 drops of aqueous lead nitrate. • To the other test-tube add 5 drops of aqueous silver nitrate. Record your observations for each test in the space below. [2] (e) Use the information in the Qualitative Analysis Notes on pages 11 and 12 to select one test to confirm the identity of the cation in FA 7 and one test to confirm the identity of the anion in FA 7. Carry out both tests and record your observations for each of the tests in the space below. Identify the ions present in FA 7. FA 7 contains … and … [3] [Total: 16]

11 9701/33/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 9701/33/M/J/11 © UCLES 2011 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, 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) 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 2011 question paper for the guidance of teachers 9701 CHEMISTRY 9701/33 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. • 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 33 © 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 PDO Recording II Appropriate headings and units for data given in weighing and accurate titration tables. Acceptable headings: mass of tube + FA1; mass of tube + residue/mass of empty tube (mass of FA1 used); initial/final or 1st/2nd (burette) (reading)/(reading at) start/finish; volume added/used/ titre; or wtte [not “difference”] Acceptable units are solidus: /cm3; brackets: (cm3); in words: volume in cubic centimeters, volume in cm3. Similarly for mass in g, etc If units are not included in the heading every entry in the table must have the correct unit. 1 PDO Recording III All accurate burette readings are given to the nearest 0.05 cm3. 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 MMO Decision IV Two uncorrected titres within 0.10 cm3 Do not allow the Rough even if ticked. Do not award this mark if having performed two titres within 0.1 cm3 a further titration is performed which is more than 0.10 cm3 from the closer of the initial two titres, unless a fourth titration, within 0.1 cm3 of any other has also been carried out. 1 Examiner rounds any burette readings to the nearest 0.05 cm3, checks subtractions and then selects the “best” titre using the hierarchy: two identical; titres within 0.05 cm3; titres within 0.1 cm3; etc to calculate mean (ignore any labelled rough). Examiner compares [corrected mean titre/corrected mass of FA 1] with Supervisor result. Calculate the ratios to 2 dp. MMO Quality Award V, VI and VII if δ ≤ 0.05 (cm3g–1) Award V and VI if 0.05 < δ ≤ 0.10 Award V only if 0.10 < δ ≤ 0.20 If the “best” titres are ≥ 0.60 cm3 apart cancel one of the Q marks. 1 1 1 [7]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 33 © University of Cambridge International Examinations 2011 (b) MMO Decision PDO Display Selects correctly subtracted accurate titre values within 0.20 cm3. Must use more than one value. If no calculation shown then titres must be indicated (e.g. with a tick) in the table Correct mean from any values selected (may include rough) by candidate given to same decimal places as most precise burette reading recorded in the table. The third decimal place may be rounded to the nearest 0.05 cm3. A mean of exactly .x25 or .x75 is allowed but the candidate may round up or down to the nearest 0.05 cm3. If ALL burette readings are given to 1 decimal place then the mean may 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 () If no working shown allow mean if value identical to that used by Examiner. 1 1 [2] (c) ACE Interpretation I In part (i) {titre from (b)/1000} × 0.01(0) If no working shown then answer must be correct. 1 II ans to (i) × 5 and ans to (ii) × 10 with no additional steps 1 III ans to (iii) × 55.8 If (iii) incorrect allow correct (ii) × 10 × 55.8 1 IV correct (ans to (iii) × 55.8/mass of FA 1) × 100 to sf shown (ecf allowed from (iii)) (sf shown may come from (i) with no previous rounding) If (iii) incorrect allow correct (ii) × 10 × 55.8 × 100/mass FA 1 (If choice of answer take the one in the answer space.) 1 PDO Display V 3 or 4 significant figures in final answers to all parts attempted (minimum three parts) 1 [5] (d) ACE Interpretation (i) Uncertainty either 1 or .5 in final place. If balance displays to 1 decimal place: error in balance reading is ±0.05 g or ±0.1(0) g If balance displays to 2 decimal places: error in balance reading is ±0.005 g or ±0.01 g If balance displays to 3 decimal places: error in balance reading is ±0.0005 g or ±0.001g 1 (ii) {2 × (i)/mass used} × 100 answer to 2, 3 or 4 sf 1 [2] [Total: 16]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 33 © University of Cambridge International Examinations 2011 2 (a) PDO Display I Tabulates mass of (empty) crucible, mass of crucible + FA 4, mass of crucible + residue/FA 4 after heating, mass of FA 4/hydrated magnesium sulfate, mass of water lost. Do not award if mass of FA 4 or mass of water incorrect 1 PDO Recording II Records all weighings consistently to at least 1 decimal place [minimum three weighings]. 1 MMO Decision III Final weighings after reheating are within 0.05 g or identical if masses recorded to 1 dp 1 Examiner to calculate [lowest mass of residue/mass of FA 4] of Supervisor and candidate to 2 dp. If two experiments carried out then use sum of masses of residues and sum of masses of FA 4 to calculate the ratio. If the Supervisor’s value is doubtful (higher than the majority of candidates) then check whether candidates are close to the expected value of 0.55 or use candidate average if majority in close agreement. (Contact team leader) MMO Quality Award IV and V if δ ≤ 0.05 Award IV only if 0.05 < δ ≤ 0.10 1 1 [5] (b) ACE Interpretation Part (i) [mass of water lost/(7 × 18)] or [mass of water lost × 246.4/18] (allow m(H2O) × 246.4/7) Mr H2O must be 18 Allow ecf for mass of water lost If two experiments carried out then mass of water may be taken from either or the mean. Allow mark if answer calculated correctly but working not shown. 1 Part (i) [mass of water lost × 246.4/7 × 18] correctly evaluated to 3 sf [= mass of water lost × 1.956] (Ignore part (ii)) There are other chemically correct methods – mark accordingly. 1 [2] (c) ACE Improvements Use a lid (for the early gentle heating) or larger mass (for smaller percentage error) or use (cool in) desiccator 1 [1] [Total: 8]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 33 © University of Cambridge International Examinations 2011 FA 5 is NaHCO3(s); FA 6 is NH4Br(s); FA 7 is H2SO4(aq) 3 (a) MMO Collection MMO Decisions On heating, steam or condensation or water vapour, misty vapour is noted or solid becomes powdery Tests for gas using limewater or in 3(d) 1 1 [2] (b) PDO Layout MMO Collection ACE Conclusion Presents results of tests in an unambiguous way Minimum 4 × 2 boxes (No reaction with cold NaOH and) gas/ammonia/fumes produced (on heating) that turn(s) red litmus blue Do not award if ppt reported with NaOH (CON) No reaction with ammonia and no reaction with barium chloride/nitrate Cream ppt with silver nitrate that partially dissolves/is insoluble in aqueous ammonia FA 6 cation: ammonium/NH4 + from some evidence and no CON obs FA 6 anion: bromide/Br– No ecf but can award Br– from any mention of cream but ppt must be present or off-white ppt insoluble or partially soluble in NH3. 1 1 1 1 1 1 [6] (c) MMO Collection Ignore any observations after water added. Steamy/misty white/orange/red/red-brown (not brown) gas/vapour/ fumes/smoke produced or gas/vapour/fumes/smoke bleaches litmus (paper) or gas/vapour/fumes/smoke turns (potassium) dichromate (solution) from orange to green 1 (White) solid turns red/orange (not yellow, not brown, not solution, not ppt) Ignore “hot” 1 ACE Conclusion FA 6 is oxidised/redox reaction/oxidation because Br– becomes Br2/Br2 is produced or redox/reduction because H2SO4 forms/becomes SO2 (with positive dichromate observation) or exothermic because tube becomes hot/heat given out. 1 [3] (d) MMO Collection Fizzing/effervescence/bubbling (occurs) (not gas is produced) If limewater test used here give second mark in (a). 1 White ppt with lead nitrate and no reaction with silver nitrate 1 [2]

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2011 9701 33 © University of Cambridge International Examinations 2011 (e) MMO Decision barium chloride/nitrate followed by hydrochloric/nitric acid (not Ba2+(aq), BaNO3, …) (If H+ already identified then “followed by hydrochloric/nitric acid” is not essential.) 1 ACE Conclusion FA 7 cation: protons/H+ if there is a positive observation with blue litmus paper/K2CrO4/Mg/Na2CO3 1 FA 7 anion: sulfate/SO4 2– Allow from minimum evidence of white ppt with Ba2+(aq) 1 [3] [Total: 16]