Cambridge A Level Chemistry 9701 — 2012 May/June Paper 3 · Variant 5

READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Give details of the practical session and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fl uid. DO NOT WRITE IN ANY BARCODES. Answer all questions. 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 10 and 11. 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 9701/35 Advanced Practical Skills 1 May/June 2012 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Confi dential Instructions UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certifi cate of Education Advanced Subsidiary Level and Advanced Level This document consists of 11 printed pages and 1 blank page. [Turn over IB12 06_9701_35/4RP © UCLES 2012 *2059021505* Session Laboratory For Examiner’s Use 1 2 3 Total

2 9701/35/M/J/12 © UCLES 2012 For Examiner’s Use You are advised to begin work on question 2, and return to question 1 later. 1 In this experiment you are to investigate the reaction between phosphoric acid and sodium hydroxide in order to determine the chemical equation. FA 1 is an aqueous solution of phosphoric acid, H3PO4. FA 2 is 0.115 mol dm–3 sodium hydroxide, NaOH. phenolphthalein indicator (a) Method Dilution ● Weigh the 100 cm3 beaker provided. Record the mass in the space below. ● Use the measuring cylinder to add about 10 cm3 of FA 1 to the beaker. Weigh the beaker with FA 1 and record the mass. ● Calculate the mass of FA 1 used and record this in the space below. ● Pour the FA 1 from the beaker into the 250 cm3 graduated (volumetric) flask provided, labelled FA 3. Wash the beaker twice with small quantities of water and add these washings to the volumetric flask. ● Make the solution up to the mark using distilled water. This diluted solution of phosphoric acid is FA 3. ● Ensure that the contents of the flask are thoroughly mixed before using FA 3 for your titrations. Titration ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add 5 drops of phenolphthalein indicator to the flask. The indicator should remain colourless. ● Titrate FA 3 with FA 2 until the indicator changes to a permanent pale pink colour. ● Perform a rough titration and record your burette readings in the space below. The rough titre is … cm3.

3 9701/35/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use I II III IV V VI VII ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure any recorded results show the precision of your practical work. ● Record in a suitable form below all of your burette readings and the volume of FA 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 have obtained this value. 25.0 cm3 of FA 3 required … cm3 of FA 2. [1] (c) Calculations Show your working and appropriate signifi cant fi gures in the fi nal answer to each step of your calculations. (i) Calculate how many moles of sodium hydroxide were present in the volume of FA 2 calculated in (b). moles of NaOH = …mol (ii) The phosphoric acid, FA 1, that you weighed out contained 8.40 % by mass of H3PO4. Calculate the mass of H3PO4 that you weighed out. mass of H3PO4 = … g (iii) Calculate how many moles of H3PO4 were present in 25.0 cm3 of the diluted solution, FA 3. (Ar : H, 1.0; O, 16.0; P, 31.0) moles of H3PO4 = … mol

4 9701/35/M/J/12 © UCLES 2012 For Examiner’s Use (iv) Use your answers to (i) and (iii) to calculate how many moles of NaOH react with 1 mole of H3PO4. Give your answer to the nearest whole number. moles of NaOH = … mol (v) When NaOH reacts with H3PO4, the salt formed could be NaH2PO4, Na2HPO4 or Na3PO4. Use your answer to (iv) to deduce which one of these three salts was the major product formed during the titration. Write the equation for the reaction of NaOH with H3PO4 to produce this salt. [5] (d) (i) A 25 cm3 pipette is accurate to ± 0.06 cm3. Calculate the maximum percentage error when the pipette was used to measure solution FA 3. percentage error in measuring FA 3 = … % (ii) State the maximum error in the mass of the 100 cm3 beaker used in the dilution of FA 1. maximum error = … g (iii) Calculate the maximum percentage error in the mass of FA 1 used. maximum percentage error = … % [2] [Total: 15]

5 9701/35/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use 2 In this experiment you are to heat a hydrated salt, FA 4, to remove the water of crystallisation. You will then calculate the relative formula mass of the anhydrous salt. The formula of FA 4 is MX2.2H2O, where M is a metal and X is a halogen. (a) Method You will carry out the following experiment twice. Record all weighings for the fi rst and second experiments in an appropriate form in the space below. ● Record the mass of an empty crucible without its lid. ● Add 2.6 – 3.4 g of FA 4 into the weighed crucible. Record the mass of the crucible and its contents. ● Use a pipe-clay triangle to support the crucible and contents on a tripod. ● Heat the crucible and its contents gently for about one minute with the lid off. Then heat strongly for a further four minutes. ● Put the lid on the crucible and leave to cool for several minutes. While you are waiting for the crucible to cool, start work on Question 1. ● When the crucible is cool, remove the lid and weigh the crucible with the anhydrous residue, MX2. ● Repeat the experiment using the second crucible. This time add 1.6 – 2.4 g of FA 4. [6] (b) Calculation Show your working and express your answers to each step to three signifi cant fi gures. (i) Calculate the number of moles of water removed from the hydrated salt in the fi rst experiment. (Ar : H, 1.0; O, 16.0) moles of H2O = … mol I II III IV V VI

6 9701/35/M/J/12 © UCLES 2012 For Examiner’s Use (ii) Complete the equation for the removal of water from hydrated FA 4. Include state symbols. MX2.2H2O(…) → …(…) + …(…) (iii) Use your equation from (ii) and your answer to (i) to calculate the number of moles of anhydrous MX2 produced in the fi rst experiment. moles of MX2 = … mol (iv) Use your results from the fi rst experiment to calculate the mass of anhydrous MX2 produced. mass of MX2 = … g (v) Calculate the relative formula mass of MX2. relative formula mass of MX2 = … [4] (c) (i) Suggest how the experimental procedure could be modifi ed to ensure that all of the water of crystallisation had been removed by heating hydrated FA 4. … … … (ii) The crucible was cooled with the lid on to prevent absorption of water vapour from the air. Suggest an alternative way of preventing water vapour being absorbed. … … [2] [Total: 12]

7 9701/35/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use 3 Qualitative Analysis At each stage of any test you are to record details of the following. ● colour changes seen ● the formation of any precipitate ● the solubility of such precipitates in an excess of the reagent added Where gases are released they should be identifi ed by a test, described in the appropriate place in your observations. You should indicate clearly at what stage in a test a change occurs. 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 reagent must be given. (a) FA 4 contains a halide ion. FA 5 is a solution of a mixture of two salts. It contains one cation and two anions. ● Place a spatula measure of FA 4 into a boiling tube. ● Half fill the boiling tube with distilled water and stir until the solid dissolves. Use this aqueous solution of FA 4 as instructed in the following tests. (i) Use aqueous silver nitrate and aqueous ammonia to identify the halide ion present in FA 4. Carry out the same test on FA 5. Use a 1 cm depth of solution in a test-tube for each test that you carry out. Record your observations and conclusions in an appropriate form in the space below. (ii) Write the ionic equation for the reaction of the halide ion in FA 4 in test (i). …

8 9701/35/M/J/12 © UCLES 2012 For Examiner’s Use (iii) Use your conclusion from 3(a)(i) and your answer to 2(b)(v) to calculate the relative atomic mass of metal M in FA 4. (Ar : F, 19.0; Cl, 35.5; Br, 79.9; I, 127) (If you were unable to calculate the relative formula mass of anhydrous MX2 in 2(b)(v) assume that it was 222, so that you are able to carry out this calculation). Ar of M = … (iv) The relative atomic masses of some of the cations on page 10 are given below. (Ar: Mg, 24.3; Ca, 40.0; Fe, 55.8; Cu, 63.5; Mn, 54.9; Zn, 65.4; Ba, 137; Pb, 207) M is one of the cations listed above. Suggest the identity of M and justify your answer. … … (v) Suggest why it was not necessary to include the cations aluminium and chromium from page 10 in the list of relative atomic masses in (iv). … … [7] (b) (i) Perform the experiments described below and record your observations in the table Record your observations in the table below. test observation To 1 cm depth of aqueous FA 4 in a test-tube, add aqueous ammonia. To 1 cm depth of FA 5 in a test-tube, add aqueous ammonia.

9 9701/35/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use (ii) Use only the results from (i) to identify the two possible cations that would have resulted in your observations for FA 4. FA 4 contains … or … (iii) Use the results from (i) to identify the cation in FA 5. FA 5 contains… (iv) Use the information on pages 10 and 11 to select one reagent, other than sodium hydroxide, to distinguish between the pair of possible cations in FA 4 identifi ed in (ii). Carry out the test with your selected reagent, using a 1 cm depth of FA 4. From this test, identify the cation in FA 4. reagent … observation … FA 4 contains … (v) Perform the tests described below and record your observations. Identify the second anion in FA 5. test observation To 1 cm depth of aqueous FA 4 in a test-tube, add 1 cm depth of FA 5. Then add 2 cm depth of dilute hydrochloric acid. FA 5 contains … [6] [Total: 13]

10 9701/35/M/J/12 © UCLES 2012 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.]

11 9701/35/M/J/12 © UCLES 2012 2 Reactions of anions ion reaction carbonate, CO3 2– CO2 liberated by dilute acids chromate(VI), CrO4 2–(aq) yellow solution turns orange with H+(aq); gives yellow ppt. with Ba2+(aq); gives bright yellow ppt. with Pb2+(aq) chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)); gives white ppt. with Pb2+(aq) bromide, Br –(aq) gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)); gives white ppt. with Pb2+(aq) iodide, I –(aq) gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)); gives yellow ppt. with Pb2+(aq) nitrate, NO3 –(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2 –(aq) NH3 liberated on heating with OH–(aq) and Al foil; NO liberated by dilute acids (colourless NO → (pale) brown NO2 in air) sulfate, SO4 2–(aq) gives white ppt. with Ba2+(aq) or with Pb2+(aq) (insoluble in excess dilute strong acids) sulfi te, SO3 2–(aq) SO2 liberated with dilute acids; gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2) chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 “pops” with a lighted splint oxygen, O2 relights a glowing splint sulfur dioxide, SO2 turns acidifi ed aqueous potassium dichromate(VI) from orange to green

12 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. 9701/35/M/J/12 © UCLES 2012 BLANK PAGE

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2012 question paper for the guidance of teachers 9701 CHEMISTRY 9701/35 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 2012 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 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total PDO layout I mass of acid used and both weighings with unit shown correctly (g), /g, mass in g or mass in grams 1 PDO recording II Rough titre shown and acceptable/appropriate headings and units for accurate titration table Minimum of 2 × 2 “boxes” Acceptable headings: initial/final or 1st/2nd (burette) (reading)/(volume)//(reading at)/(volume at) start/finish; volume added/used/titre; not “difference”, “total volume” or “volume of FA 2” Acceptable units are solidus: /cm3; brackets: (cm3); in words: volume in cubic centimeters, volume in cm3. If cm3 units are not given in the heading, every entry in the table must have the correct unit. 1 PDO recording III All accurate burette readings to 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 1 (a) MMO decision IV Two uncorrected accurate 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. Mark not awarded if any accurate reading is given to zero dp apart from initial ‘0’. 1 MMO quality Calculate candidates scaled titre = candidate mean titre × supervisor’s mass of acid/candidates mass Then compare scaled titre with the supervisor’s mean titre Award V, VI and VII if δ ≤ 0.20 cm3 Award V and VI if 0.20 < δ ≤ 0.40 cm3 Award V, only, if 0.40 < δ ≤ 0.80 cm3 Apply spread penalty as follows: titres selected (by examiner) differ by > 0.50 cm3 = –1; Apply a spread penalty of –1 if only one accurate titration is performed. 1 1 1 [7]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total (b) MMO decision Check mean titre correctly calculated from clearly selected values (ticks or working) Candidate must average two (or more) titres that are within 0.20 cm3 of each other. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Example: 26.667 must be rounded to 26.67. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp and the mean is exactly correct, e.g. 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect. Do not award this mark if: any selected titre is not within 0.20 cm3 of any other selected titre; the rough titre was used to calculate the mean; candidate carried out only 1 accurate titration; burette readings were incorrectly subtracted to obtain any of the accurate titre values used. 1 [1] (c) (i) (ii) PDO display Correct working shown in both (i) and (ii) In (i), no of moles of NaOH = 0.115 × mean volume/1000 In (ii), mass of pure H3PO4 = 0.084 × mass FA 1 weighed 1 (c) PDO display All three answers given in parts (i), (ii) and (iii) are quoted to 3 or 4 sig figs 1 (iii) ACE interpretation Correct calculation of answer to step (iii): 10 98.0 ÷ (ii) 1 (iv) ACE interpretation Ratio of moles NaOH:H3PO4 correctly calculated, to nearest integer: (ii) (i) ecf for 1:3 with mol NaOH = 0.33 Enough working must be shown to indicate that the answer was obtained by a correct method. 1 (v) ACE conclusions Correctly balanced equation, corresponding to the ratio (n) given in part (iv) If calculated value of n was not 1, 2 or 3 (when rounded to the nearest integer), then this mark cannot be awarded ecf for 1:3 “corrected” to mol NaOH = 3 If n = 1; then NaOH + H3PO4
NaH2PO4 + H2O If n = 2; then 2NaOH + H3PO4
Na2HPO4 + 2H2O If n = 3; then 3NaOH + H3PO4
Na3PO4 + 3H2O 1 [5]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total (d) (i) ACE interpret- ation % error for pipette = 0.06/25 × 100 = 0.24% (or 0.240%) 1 (ii) (iii) ACE interpret- ation No mark is allocated specifically for part (ii), but the candidate’s answer must be appropriate: if a two dp balance was used in question 1, the answer to (ii) must be 0.01 or 0.005; if a one dp balance was used, the answer to (ii) must be 0.1 or 0.05 % error in mass of FA1, given in (iii) = (2 × answer to (ii)/mass of FA 1 used )× 100 = approx 0.1% Accept numerical answer correct to 2, 3 or 4 sig fig The answer must be within +1 in final figure for 3 or 4 sf 1 [2] [Total: 15]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total MMO collection I The masses of FA 4 used by candidate were between 2.6 – 3.4 g and 1.6 – 2.4 g Award this mark based on candidate’s recorded mass of FA 4 1 PDO display II Headings for a 3 × 2 table in parallel columns or rows and the three weighings from at least one experiment must be entered in the table to qualify for this mark 1 2 (a) PDO recording III All weighings (for at least one experiment) recorded to same number of decimal places and unit, g, is given correctly 1 Examiner calculates candidate’s ratio mass loss/mass of hydrated FA 4 to 2 dp (expected ratio is 0.15) MMO quality Award IV if 0.14 ≤ ratio ≤ 0.16 in expt 1 Award V If 0.14 ≤ ratio ≤ 0.16 in expt 2 Award VI If the ratio in both of experiments 1 and 2 is between 0.12 and 0.18, inclusive 1 1 1 [6] (b) (i) (iii) PDO display Correct working shown in parts (i) and (iii), for experiment 1 In (i), there must be correct subtraction to give mass of water lost and then divided by Mr = 18 In (iii), the answer given in (i) must be divided by 2 (the mole ratio) If data from experiment 2 were used, mark ecf for this mark 1 (ii) ACE conclusion MX2.2H2O(s)
MX2(s) + 2H2O(g). 1 ACE interpretation Correct subtraction to obtain mass of anhydrous residue. Do not award this mark if data from experiment 2 were used at any point in the calculation. 1 (iv) (v) PDO display Correct use of expression: Mr = mass of residue in (iv)/no of moles in (iii) (expected answer = 208) 1 [4] (c) (i) ACE Improvements Heat the residue again and check that mass remains (almost) constant after doing so Allow “heat to constant mass” 1

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total (ii) ACE Improvements Cool in a desiccator or cool in (closed) container with a (named) drying agent 1 [2] [Total: 12]

Page 7 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total FA 4 is BaCl2; FA 5 is ZnSO4 + KI PDO layout Clear table headings and observations recorded in single table Attempts at conclusions must also be made, but they do not need to be shown in the table 1 3 (a) (i) MMO collection Mark horizontally or vertically: AgNO3 NH3 FA 4 White precipitate (ppt) soluble (in excess) FA 5 Yellow precipitate (ppt) insoluble (in excess) 1 1 (ii) ACE conclusion Ag+ + Co–
AgCl 1 (iii) ACE interpretation Correct calculation of Ar: Ar = 222 – Mr of X2 = 151 (if FA 4 was identified as chloride) Candidate may use 222 [or answer in 2(b)(v)] and 2 × Ar of the halide identified. Mark ecf if bromide was identified in FA4 from a “cream” precipitate in (i) 1 (iv) ACE conclusion Identification of M and explanation that the calculated Ar value is closest 1 (v) ACE conclusion Formula of FA 4 would be MCl3 if Cr or Al were present or Cr and Al show oxidation state +3 (or have +3 ions) (whereas M is 2+). Both ions must be discussed to earn this mark(“they” is sufficient): “no green colour” of FA 4 is acceptable to eliminate Cr3+ 1 [7] (b) (i) MMO collection FA 4 gives no change/no precipitate with NH3 FA 5 gives a white precipitate, soluble in excess ammonia 1 1 (ii) (iii) ACE conclusion FA 4 is any two of 1 (iv) MMO decision MMO collection Add sulfuric acid or potassium (di)chromate(VI) to FA 4 or suitable reagent for distinguishing between ions given in (ii) Observation for FA 4 in the test recorded correct for Ba2+ and conclusion that barium ion is present or logical conclusion from result of selected test (Expected results: White precipitate obtained with sulfuric acid or [pale] yellow precipitate with (di)chromate(VI) ions 1 1

Page 8 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 35 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark Total (v) MMO collection White precipitate, insoluble in HCl and conclusion that FA 5 contains sulfate ion 1 [6] [Total 13]