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Chemistry 9701/34 October/November 2024: question paper and mark scheme

Cambridge AS & A Level Chemistry (9701), paper 34, October/November 2024. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.

9701_w24_qp_349701_w24_ms_349701/34/O/N/24

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Paper
Paper 3
Total marks
40
Time
2 hours
Questions
3
Code on the paper
9701/34/O/N/24
9701_w24_ms_34.pdf

9701/34 mark scheme, question by question

Each question as it's printed, and the mark scheme's points for it, as text. Diagrams, tables and graphs are in the PDFs above.

Question 116 marks

Question

Iodine reacts with propanone in the presence of an acid catalyst. The rate of this reaction can be measured by determining how the concentration of iodine in the reaction mixture changes with time. CH3COCH3(aq) + I2(aq) CH3COCH2I(aq) + HI(aq) A student studies this reaction by mixing together the following three solutions and immediately starting the stop-clock. ● 100.0 cm3 of 0.1000 mol dm–3 iodine, I2 ● 50.0 cm3 of 1.00 mol dm–3 propanone, CH3COCH3 ● 50.0 cm3 of 1.00 mol dm–3 sulfuric acid, H2SO4 The student removes 25.0 cm3 of the solution. After 80 seconds, 50.0 cm3 of a solution of sodium hydrogencarbonate, NaHCO3, is added which reacts with all the sulfuric acid in the sample. Distilled water is added until the volume of the solution is 150.0 cm3. This solution is FB 1. In this experiment you will determine the concentration of iodine in FB 1 and so determine the average rate of reaction during the first 80 seconds. You will do this by titration using sodium thiosulfate solution. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) FB 1 is a sample of the solution prepared by the student. FB 2 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3. FB 3 is starch indicator. (a) Method ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 1 into a conical flask. ● Run FB 2 into the conical flask until the colour of the solution turns yellow. Then add 10 drops of FB 3. The solution will turn blue-black. ● The end-point of the titration is when the solution turns colourless. ● Perform a rough titration and record your burette readings in the space below. The rough titre is …… cm3. ● 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 all your burette readings and the volume of FB 2 added in each accurate titration. Results I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean 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] (c) Calculations (i) Give your answers to each part of (c)(ii) and (c)(iii) to an appropriate number of significant figures. [1] (ii) Use your answer to (b) to calculate the amount, in mol, of thiosulfate ions in your mean titre. amount of S2O32– = …… mol Hence calculate the amount, in mol, of iodine present in the total volume of 150.0 cm3 that the student prepares. amount of I2 = …… mol [2] (iii) Calculate the concentration, in mol dm–3, of iodine in the sample that the student removes from the reaction mixture. concentration of I2 = …… mol dm–3 [1] (iv) Calculate the initial concentration of iodine in the reaction mixture. (If you were unable to determine an answer to (c)(iii) use 0.0315 mol dm–3 as the concentration of I2 in the sample the student removed.) initial concentration of I2 = …… mol dm–3 Hence calculate the average rate of reaction during the first 80 seconds using the formula shown. change in the concentration of I2 average rate of reaction = time average rate = …… …… value units [2] (d) Suggest why the starch solution is added when the solution in the conical flask turns yellow and not added at the start of the titration. …… [1] (e) A student suggests that the experimental procedure is incorrect. The student says that the sample should be removed from the reaction mixture at 80 seconds rather than the sodium hydrogencarbonate being added at 80 seconds. State if you agree with this student. Explain your answer. …… [1] [Total: 16]

Mark scheme

1(a) I All the following data are recorded 1 • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations II Correct headings and units shown in the accurate titration table and titre values recorded for accurate titrations 1 • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FB 2 and used / added • unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded Ill All accurate burette readings are recorded to the nearest 0.05 cm3. 1 lV: The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 Award accuracy Q marks as follows: 1 V Award if δ ⩽ 0.60 cm3 VI Award if δ ⩽ 0.40 cm3 VII Award if δ ⩽ 0.20 cm3 If Supervisor’s mean titre > 50.00 cm3 then use the tolerances (0.80, 0.50, 0.30 cm3) If Supervisor’s mean titre < 10.00 cm3 then halve the tolerances (0.30, 0.20, 0.10 cm3) If Supervisor’s mean titre < 5.00 cm3 then use the tolerances (0.15, 0.10, 0.05 cm3) 1(b) Correctly calculates the mean titre to 2 decimal places. 1 • Candidate must take the average of two (or more) accurate titres that are within a total spread of not more than 0.20 cm3. • Working / explanation must be shown or ticks must be put next to the two (or more) accurate readings selected. • The mean should be quoted to 2 d.p. and be rounded to nearest 0.01 cm3. (e.g. 26.675 cm3 must be rounded to 26.68 cm3). 1(c)(i) Answers to all 3 parts of (c)(ii) and (c)(iii) quoted to 3 or 4 significant figures. 1 1(c)(ii) M1: Correctly calculates 2 amount of S2O32– = (b) / 1000 × 0.0100 (1) M2: Correct use of n(S2O32–) amount of I2 = ans / 2 × 150 / 25 (1) 1(c)(iii) Correct use of 1 amount of I2 from (c)(ii) / 0.150 or (M1 from (c)(ii) / 2) × (1000 / 25) 1(c)(iv) M1: Correctly calculates 2 0.05(00) and correctly uses (0.0500 – (c)(iii)) / 80 M2: Units mol dm–3 s–1 1(d) Release of iodine from the starch complex is slow at high iodine concentrations 1 Or A starch-iodine complex / precipitate is formed which does not decompose (during the titration) 1(e) The student is incorrect and one of: 1 • The sodium hydrogencarbonate must be added at 80 s to stop / quench the reaction (by reacting with the acid catalyst) ORA • The reaction would continue if the sodium hydrogencarbonate is not added at 80 s • The sample can be removed before 80 s as the reaction only stops when the sodium hydrogencarbonate is added. • The sodium hydrogencarbonate reacts with the acid which is the catalyst.

Question 212 marks

Question

In this experiment you will identify the ions in the hydrated salt MA2•2H2O, where M is a Group 2 metal. You will first determine the relative formula mass of the salt by measuring the mass loss when the sample is heated. Heating the sample produces the anhydrous salt and water of crystallisation. You will then select reagents to determine the identity of the ion A–. FB 4 is the salt, MA2•2H2O. (a) Method ● Weigh the empty crucible with its lid. Record the mass. ● Transfer all of FB 4 into the crucible. ● Weigh the crucible, lid and FB 4. Record the mass. ● Calculate and record the mass of FB 4 used. ● Place the crucible and contents on a pipe-clay triangle. ● Heat the crucible gently, with the lid on, for approximately 1 minute. ● Heat strongly, with the lid off, for a further 4 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. While the crucible is cooling, you may wish to begin work on Question 3. ● When the crucible has cooled, weigh the crucible with its lid and contents. Record the mass. ● Heat strongly, with the lid off, for a further 2 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. ● When the crucible has cooled, reweigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass of residue obtained. Results I II III IV V [5] (b) Calculate the amount, in mol, of water lost. amount of H2O = …… mol Hence calculate the relative formula mass, Mr, of MA2. Mr of MA2 = …… [2] (c) FB 5 is a solution of the hydrated salt MA2. The A– ion is a halide. Carry out tests to identify the halide present in MA2. Record the reagents used, the results of your tests and the identity of A–. A– is …… [2] (d) Using your answers to (b) and (c), identify the ion M2+. M2+ is …… [1] (e) A student correctly identifies A– but did not heat the sample of FB 4 for long enough to remove all the water of crystallisation. Despite this error, the student still correctly identifies M2+. Explain how the student’s answer in (b) would differ from the true answer. Explain why the student still correctly identifies M2+. [2] [Total: 12] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: ● colour changes seen ● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added ● the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme

2(a) I 4 unambiguous headings for balance readings and correct units 1 • (Mass of) crucible + lid • (Mass of) crucible, lid and FB 4 (or ‘contents before heating’) • (Mass of) crucible, lid and contents / residue after first heating • (Mass of) crucible, lid and contents / residue after second heating Units with all required headings: / g, (g), in g (or g with every entry) II Recording of all weighings 1 • All 4 weighings recorded to same number of decimal places (two or three). • Fourth weighing is within +0.02 and –0.05 g of third weighing. III Calculated masses 1 Correct subtractions to give masses of FB 4 and residue and both headings clearly labelled and both answers to min 2 sf. For assessment of accuracy (Q) marks Calculate the candidate’s mass ratio (to 2 d.p.) = mass FB 4 / mass of residue. If 4th weighing > 3rd weighing, use lower value to calculate the mass of residue. Award accuracy Q marks as follows: 2 Award IV if 1.07 ≤ ratio ≤ 1.27 Award V if 1.12 ≤ ratio ≤ 1.22 2(b) M1 Correctly calculates amount of water 2 amount of H2O = (mass of FB 4 – mass of residue) / 18 and answer to 2–4 s.f. M2 Correct use of amount of water Mr of MA2 = 2 × mass of residue / amount of H2O and answer to 2–4 s.f. 2(c) Identification of A− 2 • use of (aqueous) silver nitrate / AgNO3 • white precipitate formed • precipitate is soluble in (excess aqueous) ammonia • A− is identified as Cl − / chloride 2(d) Identification of M2+ 1 Shows working: ((b) – 71) and selects M from Group 2 with closest Ar and 2+ charge on ion 2(e) M1 (Calculated) Mr would be greater 1 and amount / moles of water is smaller or mass of residue / MA2 is greater M2 One of: 1 • there are significant / big differences between the Ar s of the elements / metals (in Group 2) owtte • most of the water was driven off / very little water of crystallisation left owtte • the calculated Ar (or Mr) is still closest to the same answer Q3 FB 6 is KMnO4; FB 7 is CuSO4(aq)

Question 312 marks

Question

(a) (i) FB 6 is potassium manganate(VII). Place all the FB 6 in a hard-glass test-tube and heat gently at first and then more strongly. Record your observations. …… [1] Leave the tube to cool and keep for use in (a)(ii). While the tube is cooling, you may wish to begin work on (b). (ii) Complete Table 3.1 by carrying out the tests described. Record your observations. Table 3.1 test observations Test 1 To a 7 cm depth of distilled water in a boiling tube, add approximately half of the residue from (a)(i). Test 2 To a 4 cm depth of aqueous sodium hydroxide in a test-tube, add the remaining residue from (a)(i). [2] (iii) Suggest what type of reaction takes place in (a)(i). …… [1] (b) FB 7 is a solution of a salt which contains a cation and an anion from those listed in the Qualitative analysis notes. FB 8 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. (i) Complete Table 3.2 by carrying out the tests described. Record your observations. Table 3.2 test observations Test 1 To a 1 cm depth of FB 7 in a test-tube, add aqueous ammonia until there is no further change, then add a few drops of hydrogen peroxide. Test 2 To a 1 cm depth of FB 7 in a test-tube, add a 1 cm depth of aqueous potassium iodide, then add FB 8. [5] (ii) The anion in FB 7 is either the sulfite ion or the sulfate ion. Carry out tests to identify which ion is present. Record the reagents used and the results of your tests. [2] (iii) Give the formula of the salt in FB 7. …… [1] [Total: 12] Qualitative analysis notes 1 Reactions of cations cation 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 warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown 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 off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(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; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white/pale yellow ppt. slowly with H+ 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 hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint 4 Tests for elements element test and test result iodine, I2 gives blue-black colour on addition of starch solution Important values, constants and standards molar gas constant R = 8.31 J K–1 mol–1 Faraday constant F = 9.65 × 104 C mol–1 Avogadro constant L = 6.022 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 s.t.p. (101 kPa and 273 K) Vm = 24.0 dm3 mol–1 at room conditions ionic product of water Kw = 1.00 × 10–14 mol2 dm–6 (at 298 K (25 °C)) specific heat capacity of water c = 4.18 kJ kg–1 K–1 (4.18 J g–1 K–1) The Periodic Table of Elements Group 1 2 13 14 15 16 17 18 1 2 H He hydrogen helium Key 1.0 4.0 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 3 4 5 6 7 8 9 10 11 12 23.0 24.3 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 at www.cambridgeinternational.org after the live examination series. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 12 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson – – – – – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 138.9 140.1 140.9 144.2 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232.0 231.0 238.0 – – – – – – – – – – – Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge

Mark scheme

3(a)(i) 2* = 1 mark 1 • (Dark) purple / purple-black solid / crystals (initially) • (Solid / crystals) jump(s) around the tube / pops / crackles (on heating) • Black powder / solid / residue (remains) • (gas) relights a glowing splint • (gas produced is) oxygen 3(a)(ii) Test 1 1 (forms) purple solution Or (water) turns purple (solution) Test 2 1 (forms) green solution or (NaOH) turns green (solution) 3(a)(iii) thermal decomposition / redox 1 3(b)(i) Observations 5 Test Observations 1 (pale) blue ppt * +NH3 (ppt) soluble / disappears in excess (NH3) or solution formed in excess (NH3) * Deep / dark blue (solution) (with excess) * black / (dark) green* + H2O2 effervescence* (gas) relights a glowing splint* 2 brown / yellow-brown / yellow solution* + KI (off-)white / (pale) brown / yellow-brown precipitate / solid* + Solution / solid / ppt becomes paler owtte* Na2S2O3 Precipitate partially dissolves or ppt dissolves in excess (thiosulfate)* 3(b)(ii) M1 Choice of reagent(s) 1 Selects barium chloride / BaCl2 or barium nitrate / Ba(NO3)2 and nitric acid / HNO3 or hydrochloric acid / HCl OR Selects (acidified aqueous) potassium manganate(VII) / (H+) & KMnO4 M2 Clear results to show: 1 white ppt (with Ba2+) and insoluble in acid OR (KMnO4) remains purple / not decolourised / no change 3(b)(iii) CuSO4 1

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Chemistry 9701/34 for October/November 2024 is out of 40 marks, with 2 hours to answer it, across 3 questions.

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