Cambridge A Level Chemistry 9701 — 2010 Oct/Nov Paper 3 · Variant 6

This document consists of 12 printed pages. DC (LEO) 28255/3 © UCLES 2010 [Turn over * 4 9 9 1 5 5 1 7 1 2 * 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 9701/36 Advanced Practical Skills October/November 2010 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Instructions to Supervisors Session Laboratory For Examiner’s Use 1 2 3 Total

2 9701/36/O/N/10 © UCLES 2010 For Examiner’s Use You must prepare Flask A and Flask B in Question 2 before starting Question 1. Shake each flask periodically during the time you spend on Question 1. 1 FB 1 is 0.125 mol dm–3 sulfuric acid, H2SO4. FB 2 is an aqueous solution of sodium hydroxide, NaOH. You are to determine the concentration, in mol dm–3, of the sodium hydroxide in FB 2. (a) Method • Fill a burette with FB 1. • Run between 45.50 cm3 and 46.50 cm3 of FB 1 from the burette into the 250 cm3 graduated (volumetric) flask, labelled FB 3. • Make up to the mark with distilled water. • Shake the flask to mix the solution. In the space below record your burette readings and the volume of FB 1 added to the graduated flask. You are reminded to shake Flask A and Flask B periodically. Titration • Fill a second burette with FB 2. • Pipette 25.0 cm3 of FB 3, the diluted acid, into a conical flask. • Add to the flask a few drops of phenolphthalein indicator. • Place the flask on a white tile. • Titrate the acid in the flask with FB 2. At the end-point a “permanent” pink colour will remain in the solution. • Note: The “permanent” pink colour will fade over several minutes as carbon dioxide is absorbed from the atmosphere. 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 on page 3 all of your burette readings and the volume of FB 2 added in each accurate titration. • Make certain any recorded results show the precision of your practical work. You will require the burette containing FB 2 for Question 2.

3 9701/36/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use [7] (b) From your accurate titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value. 25.0 cm3 of FB 3 required … cm3 of FB 2. [1] Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (c) (i) Calculate how many moles of H2SO4 in FB 1 were run from the burette into the 250 cm3 graduated, (volumetric) flask. … mol of H2SO4 were run from the burette into the graduated flask. (ii) Calculate how many moles of H2SO4 in FB 3 were pipetted from the graduated flask into the conical flask in each titration. … mol of H2SO4 were pipetted into the conical flask. (iii) Calculate how many moles of NaOH reacted with the H2SO4 in (ii). H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) The H2SO4 in the titration flask reacted with … mol of NaOH. I II III IV V I II III IV V VI VII

4 9701/36/O/N/10 © UCLES 2010 For Examiner’s Use (iv) Calculate the concentration, in mol dm–3, of NaOH in FB 2. The concentration of NaOH in FB 2 is …………………. mol dm–3. [5] (d) The individual error in any burette reading is ±0.05 cm3. Two students, A and B, record identical burette readings. final burette reading 25.60 cm3 initial burette reading 1.35 cm3 volume added 24.25 cm3 Explain the following. (i) The initial burette reading made by student A was 0.05 cm3 greater than the true value but the volume added was exactly 24.25 cm3. … … (ii) The initial burette reading made by student B was 0.05 cm3 less than the true value and the actual volume added was exactly 24.15 cm3. … … [2] (e) In the instructions for the experiment you were told that the “permanent” pink colour at the end-point would fade over a few minutes as carbon dioxide is absorbed from the atmosphere. (i) Explain why absorption of carbon dioxide at the end-point would reverse the indicator colour change seen in the titration. … … (ii) Suggest a modification to the titration method, using the same indicator, that would overcome this problem. … … [2] [Total: 17]

5 9701/36/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use 2 FB 4 is 0.050 mol dm–3 sodium hydroxide solution. FB 5 is 0.200 mol dm–3 propanoic acid, C2H5CO2H. FB 6 is an organic liquid that does not mix with water. Propanoic acid dissolves both in water and in the organic layer, FB 6. When an aqueous solution of the acid is shaken with FB 6, some of the acid transfers to the organic layer. The amount of acid remaining in the aqueous layer can be determined by titration with aqueous sodium hydroxide. Preparation of the mixture in Flask A and in Flask B. Flask A • Use a measuring cylinder to place 50 cm3 of FB 5 into the stoppered flask labelled Flask A. • Use a second measuring cylinder to add to the flask 40 cm3 of FB 6, the organic liquid. • Replace the stopper in the flask. Flask B • Use the first measuring cylinder to place 50 cm3 of FB 5 into the stoppered flask labelled Flask B. • Use the second measuring cylinder to add to the flask 60 cm3 of FB 6, the organic liquid. • Replace the stopper in the flask. • Shake both flasks vigorously for about 1 minute. • Leave the flasks on the workbench and start Question 1. • Shake the flasks for a further minute at intervals during the course of your work on another question. (a) Titrations For each flask follow the same procedure. • Empty the burette containing FB 2. • Rinse the burette thoroughly with FB 4. • Fill the burette with FB 4. • Ensure the two layers have separated – this should take no longer than 1 minute after shaking the flask. • Pipette 10.0 cm3 of the lower (aqueous) layer into a conical flask. Attach the pipette filler to the pipette before inserting it into the mixture, in order to close the top of the pipette to prevent any of the top (organic) layer from entering the pipette. • Replace the stopper in the flask. • Titrate the acid in the conical flask with FB 4, using phenolphthalein indicator, as in Question 1. • One titration will be sufficient for each experiment but take care to ensure that no errors are made during the procedure. I II III IV

6 9701/36/O/N/10 © UCLES 2010 For Examiner’s Use Results Record, in a single table below, the burette readings and volume of FB 4 added, for each of Flask A and Flask B. [4] (b) Calculations In these calculations make use of the following. • The concentration of NaOH in FB 4 is 0.050 mol dm–3. • 50 cm3 of 0.200 mol dm–3 propanoic acid, the volume of acid added to each flask, contains 0.010 mol C2H5CO2H. • 1 mol C2H5CO2H reacts with 1 mol NaOH. (i) Calculate the volume of FB 4 that contains 0.010 mol NaOH. This is the volume of FB 4 that would have reacted with the propanoic acid in the 50 cm3 of the aqueous layer, before shaking with the organic liquid. Volume of FB 4 = … cm3 (ii) For each flask, use your titration result in (a) to calculate the volume of FB 4 needed to react with the acid remaining in 50 cm3 of the aqueous layer, after shaking with the organic liquid. Flask A Flask B volume of FB 4 = … cm3 volume of FB 4 = … cm3 (iii) The amount of propanoic acid transferred to the organic layer can be represented by the following. (answer to (i) – answer to (ii)) For each flask evaluate this expression. Flask A (answer to (i) – answer to (ii)) = … cm3 Flask B (answer to (i) – answer to (ii)) = … cm3 [2]

7 9701/36/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use (c) In which flask was most propanoic acid transferred to the organic layer? Justify your answer. … … … [1] (d) It is suggested that shaking the mixture leads to the following equilibrium being established. C2H5CO2H(aq) C2H5CO2H(org) Determine the equilibrium constant by evaluating the expressions in the following table. (i) Determine the equilibrium constant by evaluating the expressions in the following table. Ignore units. Flask A Flask B Kc = answer (b)(iii) answer (b)(ii) × 1.25 Kc = answer (b)(iii) answer (b)(ii) × 0.83 Kc = … Kc = … (ii) Explain whether or not your results support the idea that equilibrium has been established in each flask. … … … [1] [Total: 8]

8 9701/36/O/N/10 © UCLES 2010 For Examiner’s Use 3 FB 7, FB 8 and FB 9 are aqueous solutions, each containing cations and anions from those listed on pages 11 and 12 in the Qualitative Analysis Notes. 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. (a) (i) One or more of the solutions FB 7, FB 8 and FB 9 are believed to contain the ammonium ion, NH4 +. Suggest a reagent that would enable you to identify the presence of NH4 + and describe how you would use the reagent in an appropriate test. reagent … test … … Use this reagent to test each of the solutions. Record your observations in the table below. solution observation FB 7 FB 8 FB 9 I II

9 9701/36/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use (ii) One or more of the solutions contains the sulfate ion, SO4 2–. Select reagents that would enable you to identify the presence of SO4 2–. Show clearly, by describing how the reagents will be used, how you would distinguish SO4 2– from the sulfite ion, SO3 2–. reagents … test … Use these reagents to test each of the solutions. Record your observations in the table below. solution observation FB 7 FB 8 FB 9 (iii) Conclusions The ammonium ion, NH4 +, is present in … The sulfate ion, SO4 2–, is present in … [5] (b) Use aqueous sodium hydroxide and aqueous ammonia in separate tests to identify any cation (apart from NH4 +) present in FB 7, FB 8 and FB 9. Present your results for each of the solutions in a suitable form below. [4] III IV V I II III IV

10 9701/36/O/N/10 © UCLES 2010 For Examiner’s Use (c) Conclusion Complete the following table. Place a cross in any box where no cation has been identified. solution cation supporting evidence FB 7 FB 8 FB 9 [1] (d) Carry out the following tests on FB 10. Observe carefully at each stage and record all of your observations in the table. test observations (i) Place 2 spatula measures of FB 10 in a dry, hard glass boiling-tube. Heat the solid gently at first, then strongly until no further change is seen. Retain the solid for use in (ii). (ii) Tip the contents of the tube in (i) into a second boiling-tube. Add 2 cm depth of dilute hydrochloric acid a little at a time. Warm the tube and leave to stand. [5] [Total: 15] I II III IV V

11 9701/36/O/N/10 © UCLES 2010 [Turn over Qualitative Analysis Notes Key: [ ppt. = precipitate ] 1 Reactions of aqueous cations ion reaction with NaOH(aq) NH3(aq) aluminium, Al3+(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 9701/36/O/N/10 © UCLES 2010 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 acid) sulfite, SO3 2– (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, 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 October/November 2010 question paper for the guidance of teachers 9701 CHEMISTRY 9701/36 Paper 3 (Advanced Practical Skills), 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 October/November 2010 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 – October/November 2010 9701 36 © UCLES 2010 Question Sections Indicative material Mark 1 (a) PDO layout I Volume given for Rough titre and accurate titre details tabulated. 1 MMO Collection II Follows instructions - dilutes 45.50 – 46.50 cm3 FB 1 and initial and final burette readings recorded for Rough titre and initial and final burette readings and volume of FB 2 added recorded for each accurate titre 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 MMO Decisions III Has two uncorrected, accurate titres within 0.1 cm3 Do not consider 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 the third titration (or first two) has also been carried out. 1 PDO Recording IV All accurate burette readings (initial and final) recorded to nearest 0.05 cm3 (Accurate titration & dilution tables) Assess this mark on burette readings only 1 For candidates and Supervisor scale titre for 46.00 cm3 FB 1 diluted. Calculate titre × added FB1 of volume 46.00 Calculate difference in Supervisor and candidate scaled values and award “quality” marks as below. [If candidate has not recorded a volume diluted, use 46.00 cm3]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – October/November 2010 9701 36 © UCLES 2010 MMO Quality V, VI and VII Round any burette readings to the nearest 0.05 cm3. Check and correct subtractions in the titre table. Select the “best” titre using the hierarchy: two identical; titres within 0.05 cm3; titres within 0.1 cm3; etc. Award V, VI and VII for a difference from Supervisor within 0.20 cm3 Award V and VI only for a difference of 0.20+ cm3 – 0.30 cm3 Award V only for a difference of 0.30+ – 0.50 cm3 If the “best” titres are ≥ 0.50 cm3 apart cancel one of the Q marks. 3 [7] (b) ACE Interpretation Calculates the mean, correct to 2 decimal places (third decimal place rounded to the nearest 0.05 cm3) from any accurate titres within 0.20 cm3. A mean of exactly .×25 or .×75 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 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. 1 [1] (c) ACE Interpretation I Expression correct in step (i) 1000 diluted volume × 0.125 II Uses answer to (i) × 250 25 in step (ii) III Uses answer to (ii) × 2 in step (iii) and answer to (iii) × titre 1000 in step (iv) If an answer, with no working, is given in any section allow if correct. 1 1 1 PDO Display IV Appropriate working shown in a minimum of 3 sections. 1 V 3 to 5 significant figures in final answers to all sections attempted – minimum of 3 final answers required to qualify for the award of this mark. 1 [5]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – October/November 2010 9701 36 © UCLES 2010 (d) ACE Interpretation (i) For Student A explains that final burette reading was also 0.05 cm3 greater than the true value (“error” in same direction) Ignore parallax error Not errors cancel – reason needed (ii) For Student B explains that final burette reading was 0.05 cm3 greater than the true value (“error” in opposite direction) Not errors compound each other/add up 1 1 [2] (e) ACE Conclusions ACE Improvements (i) Explains that carbon dioxide is acidic (and its absorption reverses the colour change of the indicator) (ii) Puts acid/FB 3 in burette and pipettes NaOH/ FB 2 into flask or Heat the solution/Use hot solution 1 1 [2] [Total: 17]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – October/November 2010 9701 36 © UCLES 2010 2 (a) PDO Recording MMO Quality I Records results in a single table for both experiments. No repetition of headings II Titre for either Flask A or B within 0.50 cm3 of Supervisor III Titre for either Flask A or B within 0.30 cm3 of Supervisor IV Titre for both Flask A and B within 0.30 cm3 of Supervisor 1 1 1 1 [4] (b) ACE Interpretation (i) Calculates a volume of 200 cm3 in step (i) (ii) Correctly calculates titre x 5 for each flask 1 1 [2] (c) ACE Conclusions Mark consequentially to practical results Chooses expt with lower titre – less remains (or converse argument) or higher value in (b)(iii) Allow ecf 1 [1] (d) ACE Conclusions Comparison of candidate’s Kc values Judgement on constancy or otherwise Supports/does not support equilibrium 1 [1] [Total: 8]

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – October/November 2010 9701 36 © UCLES 2010 FB 7 is Fe(NH4)2(SO4)2(aq); FB 8 is Na2SO4(aq); FB 9 is CaCl2(aq) 3 (a) MMO Decisions MMO Collection MMO Decisions MMO Collection MMO Conclusions I Selects sodium hydroxide as reagent (Not if + Al) and describes (warming the solution and) testing any gas evolved with red litmus/pH paper II Records positive test for ammonia gas with FB 7 only Must link gas/NH3 with positive test (Allow even if Al mentioned in I) III Selects barium chloride or nitrate together with HCl or HNO3 Do not accept Ba2+ as a reagent Accept Ba2+(aq) or a solution containing Ba2+ ions IV White ppt, persisting in acid with FB 7 and with FB 8 Allow from unspecified strong acid provided there is no ppt with FB 9. V Mark consequentially to observations for solutions containing NH4 + and SO4 2- ecf allowed here but not with other identities Allow from strong acid or from H2SO4 if clearly added after Ba2+(aq) 1 1 1 1 1 [5] (b) PDO Layout MMO Collection I (Tabulates) observations clearly, showing: observation when each reagent is first added and observation when reagent added to excess if there is a ppt II, III and IV 1 mark for correct observations in each of the columns or rows representing FB 7, FB 8 and FB 9 or 1 mark for correct observations in the row or column representing a reagent added (initial and excess count as one row/column) 1 3 [4] Minimum observations Solution FB 7 FB 8 FB 9 NaOH Green ppt insoluble (in excess) no reaction/no change/no ppt Not “–“ words needed (Only penalise once) White ppt insoluble (in excess) NH3 Green ppt insoluble (in excess) colourless soln/no reaction/no change/no ppt No reaction/no change/no ppt

Page 7 Mark Scheme: Teachers’ version Syllabus Paper GCE A/AS LEVEL – October/November 2010 9701 36 © UCLES 2010 (c) ACE Conclusions One mark for FB 7 and FB 9 containing Fe2+ and Ca2+ respectively provided no CON obs in (a) or (b) No ecf Ignore FB 8, ignore supporting evidence 1 [1] FB 10 is CuCO3(s) (d) (i) (ii) MMO Collection MMO Decisions MMO Collection I observes the solid turning black in step (i) II observes fluidity in solid layer in step (i) Allow description of fluidised solid as “liquid” III describes an appropriate test for any of the following gases: O2, CO2, NH3 or SO2 (gas or O2/etc needed) IV lime water turns milky/cloudy/chalky Gas or CO2 turns limewater milky. scores III and IV V on adding acid to residue from FB 10, observes green solution (on warming) Ignore any residual solid Allow blue-green or bluish green Allow if (qualified) green solution turns blue on cooling May award either III or IV here but only for gas tests for CO2 or SO2 or limewater observations 1 1 1 1 1 [5] [Total: 15]