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Chemistry 9701/31 May/June 2025: question paper and mark scheme

Cambridge AS & A Level Chemistry (9701), Paper 3 Advanced Practical Skills 1, May/June 2025. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.

9701_s25_qp_319701_s25_ms_319701/31/M/J/25

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Paper
Paper 3 Advanced Practical Skills 1
Total marks
40
Time
2 hours
Questions
3
Code on the paper
9701/31/M/J/25
9701_s25_ms_31.pdf

9701/31 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 110 marks

Question

You will determine the percentage purity of a sample of calcium carbonate by reacting it with excess hydrochloric acid and measuring the volume of carbon dioxide produced. CaCO3(s) + 2HCl (aq) CaCl 2(aq) + H2O(l) + CO2(g) FA 1 is impure calcium carbonate, CaCO3. FA 2 is 0.500 mol dm–3 hydrochloric acid, HCl . (a) Method • Weigh the container with FA 1. Record the mass in the space for results. • Fill the tub with water to a depth of approximately 5 cm. • Fill the 250 cm3 measuring cylinder completely with water. Holding a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. • Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub. • Using the 50 cm3 measuring cylinder, transfer 50.0 cm3 of FA 2 into the flask labelled X. Check that the bung fits tightly into the neck of flask X, clamp flask X and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. • Remove the bung from the neck of the flask. Tip all of the FA 1 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. • Replace the flask in the clamp and leave until the fizzing stops. Swirl the flask occasionally. • Weigh the container with any residual FA 1. Record the mass. • Calculate and record the mass of FA 1 that is added to the acid. • When no more gas is collected, record the final volume of gas. You may wish to start Question 2 or Question 3 while the gas is being collected. Results I II III [3] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide collected in the measuring cylinder (at room conditions). amount of CO2 = …… mol Hence, deduce the amount, in mol, of calcium carbonate present in the FA 1 used. amount of CaCO3 = …… mol [1] (ii) Use your answer to (b)(i) and the mass of FA 1 used to calculate the percentage purity of calcium carbonate. Show your working. purity of calcium carbonate = …… % [1] (c) A student carries out the experiment described in (a) using 25.0 cm3 of 1.00 mol dm–3 hydrochloric acid. The mass of FA 1 is not changed. Suggest the consequence of this change on the percentage purity of calcium carbonate calculated by placing one tick (3) in Table 1.1. Table 1.1 The percentage purity calculated would increase. The percentage purity calculated would stay the same. The percentage purity calculated would decrease. Explain your answer. [2] (d) The student repeats the method in (a) but the hydrochloric acid is not used in excess. (i) State an observation that shows that the acid is not used in excess. …… [1] (ii) Explain what effect, if any, this would have on the percentage purity of calcium carbonate calculated in (b)(ii). …… [2] [Total: 10]

Mark scheme

1(a) I Unambiguous headings in space for results AND correct units in the space for results for: 1 • (mass of) container with FA 1 / solid • (mass of) container (+ residual FA 1 / solid) • (mass of) FA 1 • (volume of) gas / CO2 (produced) II Both weighings to a consistent number of decimal places (either to 2 dp or to 3 dp) 1 AND mass of FA 1 correctly calculated AND volume of gas (in cm3) produced recorded as an integer value (No headings are required for this mark.) III Volume of gas produced 1 Award in range 90–210 cm3 (0.090–0.210 dm3) 1(b)(i) Correctly calculates 1 (amount of CO2 =) volume of gas collected / 24000 mol AND amount of CaCO3 = amount of CO2 AND both answers to 2–4 sf 1(b)(ii) Correct display 1 % purity = [(b)(i) x 100.1/mass of FA 1] × 100 AND answer to 2–4 sf 1(c) M1: the bottom box is ticked 2 AND the volume of gas collected is less M2: more gas escapes (before the bung is replaced) AND the (initial) rate is greater 1(d)(i) One of: 1 • unreacted solid / FA 1 / CaCO3 remains in the (conical) flask (after bubbling stops) • not all the solid / FA 1 has reacted / disappeared (at end of reaction) 1(d)(ii) % purity is less AND 2 M1: volume of gas collected is less % purity is less AND M2: so amount / moles / mass of calcium carbonate (calculated) is lower

Question 218 marks

Question

You will determine the percentage purity of another sample of calcium carbonate, CaCO3, by titration. The experiment involves three steps. step 1 A known mass of the same impure calcium carbonate, FA 1, is reacted with an excess of hydrochloric acid to form FA 3. This step has been done for you. step 2 You will dilute the products of step 1 to a known volume. step 3 You will carry out a titration to find out how much acid remains after the reaction in step 1. FA 3 has been prepared by reacting 19.0 g of FA 1 with 250 cm3 of 2.00 mol dm–3 hydrochloric acid, HCl . FA 5 is 0.0900 mol dm–3 sodium hydroxide, NaOH. FA 6 is bromophenol blue indicator. (a) Method step 2 • Pipette 25.0 cm3 of FA 3 into the 250 cm3 volumetric flask. • Make the solution up to 250 cm3 with distilled water. • Thoroughly mix the contents of the volumetric flask. This solution is FA 4. step 3 • Fill the burette with FA 5. • Rinse the pipette with distilled water and then with FA 4. • Pipette 25.0 cm3 of FA 4 into a conical flask. • Add several drops of FA 6. • 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, in a suitable form in the space below, all your burette readings and the volume of FA 5 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 use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FA 4 required …… cm3 of FA 5. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium hydroxide in the volume of FA 5 in (b). amount of NaOH = …… mol [1] (iii) Use your answer to (c)(ii) to calculate the amount, in mol, of hydrochloric acid in 250 cm3 of FA 4. amount of HCl in 250 cm3 of FA 4 = …… mol Hence, calculate the amount, in mol, of hydrochloric acid present in 250 cm3 of FA 3. amount of HCl in 250 cm3 of FA 3 = …… mol [1] (iv) Calculate the amount, in mol, of hydrochloric acid used to prepare FA 3. amount of HCl = …… mol [1] (v) Use your answers to (c)(iii) and (c)(iv) to calculate the amount, in mol, of hydrochloric acid that reacted with the FA 1 in step 1. amount of HCl that reacted with FA 1 = …… mol [1] (vi) Use your answer to (c)(v) to calculate the amount, in mol, of calcium carbonate present in FA 1. CaCO3(s) + 2HCl (aq) CaCl 2(aq) + H2O(l) + CO2(g) amount of CaCO3 in FA 1 = …… mol [1] (vii) Use your answer to (c)(vi) and the mass of FA 1 used to calculate the percentage purity of calcium carbonate. Show your working. purity of calcium carbonate = …… % [1] (d) The percentage purity of calcium carbonate in FA 1 has been determined by two different methods, gas collection and titration. The gas collection method used in Question 1 is less accurate. (i) Suggest two reasons why the gas collection method is less accurate. 1 …… 2 …… [2] (ii) Describe a change to the gas collection method that would improve the accuracy of the percentage purity determined in Question 1. …… [1] [Total: 18] 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 The following data must be shown 7 • two burette readings AND titre for rough titration • initial and final burette readings for two (or more) accurate titrations II Titre values recorded for accurate titrations AND correct headings and units in the accurate titration table • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre OR volume / FA 5 AND used / added unit: / cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded III All accurate burette readings are to nearest 0.05 cm3 IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre. Accuracy (Q) marks: marks V, VI and VII will be determined by comparison of candidate and supervisor Round burette readings to the nearest 0.05 cm3. Check and correct titre subtractions where necessary. Select the best mean titre, using the following hierarchy: • 2 identical accurate titres (ignoring any that are labelled ‘rough’), then • accurate titres within 0.05 cm3, then • accurate titres within 0.10 cm3, etc. Calculate the Supervisor’s mean titre to two decimal places. Calculate the candidate’s mean titre to two decimal places. Calculate the difference (δ) between the supervisor’s and candidate’s mean titre. V, VI, VII Award V if δ ⩽ 0.80 cm3 Award VI if δ ⩽ 0.50 cm3 Award VII if δ ⩽ 0.30 cm3 2(b) Correct calculation of the mean titre 1 • candidate must take the average of two (or more) 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 must be quoted to 2 decimal place and be rounded to the nearest 0.01 cm3, e.g., 26.675 must be rounded to 26.68. 2(c)(i) Significant figures 1 All quoted answers in (c)(ii)–(c)(v) are expressed to 3 or 4 sf. 2(c)(ii) Correctly calculates 1 amount of NaOH = 0.09 × (vol (b)/1000) mol 2(c)(iii) Correctly calculates 1 amount of HCl in 250 cm3 FA 4 = (c)(ii) × 10 mol AND amount of HCl in 250 cm3 FA 3 = [(c)(ii) × 10] × 10 mol 2(c)(iv) Correctly calculates 1 amount of HCl used to prepare FA 3 = (250/1000) × 2 = 0.500 mol 2(c)(v) Correctly uses (c)(iv) – (c)(iii) 1 2(c)(vi) Correctly uses 1 amount of CaCO3 present = (c)(v)/2 mol 2(c)(vii) Correctly uses 1 % purity = [(c)(vi) × 100.1 / 19.0] × 100 2(d)(i) Any two of: 2 • some gas / carbon dioxide dissolves in the water (in trough / measuring cylinder) • some gas escapes before the bung is replaced on the conical flask • the impurity may react to produce gas 2(d)(ii) Any one of: 1 • description of both CaCO3 and acid being separated within the stoppered flask until reaction starts, e.g. use of small container for CaCO3 which can be tipped over, package of CaCO3 held by a thread, etc. • saturate the water with CO2 prior to experiment / use of warmer water • use of gas syringe FA 7 is NH4Cl and CuCl2; FA 8 is NaHCO3; FA 9 is Na2CO3

Question 312 marks

Question

(a) FA 7 is a solution containing two cations and one anion. All of the ions are listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Tables 3.1 and 3.2. For each test use a 1 cm depth of FA 7 in a test-tube unless the solution requires heating. Table 3.1 test observations Test 1 Add aqueous ammonia. Test 2 To a 1 cm depth of FA 7 in a boiling tube, add aqueous sodium hydroxide, then warm the mixture carefully. Test 3 Add a few drops of silver nitrate. Table 3.2 test observations Test 4 Add a 1 cm depth of aqueous barium chloride or aqueous barium nitrate. Test 5 Add a 1 cm depth of dilute nitric acid, then leave to stand for a few minutes. Test 6 Add a 1 cm depth of aqueous potassium iodide, then add a few drops of starch solution. [5] (ii) Write an ionic equation for a reaction in (a)(i) that resulted in the formation of a precipitate. Include state symbols. …… [1] (iii) Give the formula of each ion present in FA 7. If you cannot identify an ion, write ‘unknown.’ cations …… and …… anion …… [2] (b) FA 8 is an anhydrous sodium compound. (i) Transfer a spatula measure of FA 8 to a hard-glass test-tube. Heat the test-tube gently at first, then more strongly until no further change occurs. Record your observations. …… [1] (ii) The FA 9 provided is a sample of the residue obtained from heating FA 8. To a 2 cm depth of nitric acid in a test-tube, slowly add a small spatula measure of FA 9. Record your observations. …… [2] (iii) Deduce the formula of FA 8. FA 8 is …… . [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), off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown Mn2+(aq) 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 F = 9.65 × 104 C mol–1 Faraday constant Avogadro constant L = 6.02 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 at 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 23.0 24.3 3 4 5 6 7 8 9 10 11 12 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 16 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 (round down) 5 Test 1 (pale) blue ppt * soluble in excess * (to give a) dark / deep blue solution * NH3(aq) Test 2 (pale) blue ppt AND insoluble in excess * NaOH(aq) warm gas / bubbles / NH3 turns (red) litmus blue * black solid / ppt / residue * Test 3 white ppt * AgNO3 Test 4 no (visible) change / remains pale blue solution / no (visible) reaction / no Ba2+(aq) ppt * Test 5 no (visible) change / remains pale blue solution / no (visible) reaction / no Dilute HNO3 effervescence AND standing no change / remains pale blue solution * Test 6 Brown * KI(aq) starch Blue-black / dark blue / black * 3(a)(ii) Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) 1 OR Ag+(aq) + Cl–(aq) → AgCl(s) OR 2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I2(aq) 3(a)(iii) Cations: Cu2+, NH4+ 2 Anions: Cl– 2 correct = 1 mark. All 3 correct = 2 marks 3(b)(i) condensation / water vapour / (colourless) droplets / steam 1 3(b)(ii) M1: effervescence / bubbling / fizzing 2 M2: EITHER gas / fizzing / CO2 gives a white ppt with limewater OR (white solid / FA 9 reacts and) colourless solution formed 3(b)(iii) FA 8 is NaHCO3 1 from appropriate observations: condensation in (b)(i) AND evidence of CO2 in (b)(ii)

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On this page, free: the mark scheme for AS & A Level Chemistry 9701/31, May/June 2025 (9701_s25_ms_31), and the question paper (9701_s25_qp_31), to read or download, with each question's marking points written out underneath.

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Chemistry 9701/31 for May/June 2025 is out of 40 marks, with 2 hours to answer it, across 3 questions.

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