Cambridge A Level Chemistry 9701 — 2025 May/June Paper 3 · Variant 4

1 Iron is an element that is essential in the human diet. Some people need to take iron supplement tablets to ensure an adequate intake of iron. You will investigate the mass of iron in an iron supplement tablet by titrating a solution with potassium manganate(VII). Fe2+(aq) Fe3+(aq) + e– MnO4–(aq) + 8H+(aq) + 5e– Mn2+(aq) + 4H2O(l) FB 1 is an aqueous solution of iron supplement tablets made by dissolving 14 tablets in 1.00 dm3 of solution. The iron in each tablet is iron(II) sulfate, FeSO4•7H2O. FB 2 is 0.0100 mol dm–3 acidified aqueous potassium manganate(VII), KMnO4. FB 3 is dilute sulfuric acid, H2SO4. (a) Method • Fill a burette with FB 2. • Pipette 25.0 cm3 of FB 1 into a conical flask. • Use the 25 cm3 measuring cylinder to add 10.0 cm3 of FB 3 to the conical flask. • 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 FB 2 added in each accurate titration. Rinse the burette with distilled water and leave to drain while you continue Question 1. 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 FB 1 required .............................. cm3 of FB 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of manganate(VII) ions in the volume of FB 2 in (b). amount of MnO4– = .............................. mol [1] (iii) Use your answer to (c)(ii) and the equations at the start of the question to calculate the concentration, in mol dm–3, of iron(II) ions in FB 1. concentration of Fe2+ = .............................. mol dm–3 [1] (iv) Use your answer to (c)(iii) to calculate the concentration, in g dm–3, of iron(II) ions in FB 1. concentration of Fe2+ = .............................. g dm–3 [1] (v) The manufacturer of the iron supplement tablets used to make FB 1 claims that each tablet contains a minimum of 150 mg of Fe2+. Use your answer to (c)(iv) and the information given about FB 1 to determine whether this claim is correct. Show your working. [1] (d) A student used all the FB 3 and suggests that dilute hydrochloric acid would be a suitable replacement. Suggest whether the student is correct or not. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total:14]

Mark scheme: Question Answer Marks 1(a) l All the following data recorded. 7 • two burette readings AND titre for the rough titration • initial and final 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 used / added OR FB 2 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 recorded to 0.05 cm3. IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre. Accuracy (Q) marks 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: • two (or more) accurate identical titres (ignoring any that are labelled “rough”), then • two (or more) accurate titres within 0.05 cm3, then • two(or more) 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 () between the Supervisor’s and Candidate’s mean titre. Award accuracy Q marks as follows: V award if  ⩽ 0.50 cm3 Vl award if  ⩽ 0.30 cm3 Vll award if  ⩽ 0.20 cm3 1(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 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) Final answers for (c)(ii), (c)(iii) and (c)(iv) to 3 or 4 significant figures. 1 1(c)(ii) Correctly calculates 1 amount of manganate(VII) = 0.01  volume in (b) /1000 mol 1(c)(iii) Correctly uses 1 amount of Fe2+ reacting with FB 2 = (c)(ii)  5 mol AND concentration of Fe2+ = ((c)(ii)  5)  40 mol dm−3 1(c)(iv) Correctly uses 1 concentration of Fe2+ = (c)(iii)  55.8 g dm-3 1(c)(v) Conclusion 1 yes / correct (because) AND (c)(iv)  1000 / 14 ( 150 claimed as present in tablet) 1(d) no because chloride (ions) are oxidised by manganate(VII) / KMnO4. 1 OR no because hydrochloric acid is monoprotic AND might not produce enough H+. OR yes because hydrochloric acid is a strong acid.

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2 The reaction between an acid and an alkali is exothermic. You will carry out a neutralisation experiment to determine the enthalpy change involved. You will mix different volumes of an acid with a fixed volume of an alkali and measure the temperature rises that occur. FB 4 is aqueous sodium hydroxide, NaOH. FB 5 is 2.00 mol dm–3 hydrochloric acid, HCl . (a) Method • Use the thermometer to measure the initial temperature of FB 4. Record this initial temperature in the space for results. • Support the cup in the 250 cm3 beaker. • Fill one burette with FB 5. Label the burette FB 5. • Fill the other burette with distilled water. Experiment 1 • Use the 10 cm3 pipette to transfer 10.0 cm3 of FB 4 into the cup. • Add 9.00 cm3 of distilled water from the burette to the same cup. • Add 1.00 cm3 of FB 5 from the other burette to the same cup. • Stir the mixture and use the thermometer to measure the maximum temperature. If necessary, tilt the cup so that the solution covers the bulb of the thermometer. • Record the maximum temperature in Table 2.1. • Empty, rinse and dry the cup ready for use in further experiments. Further experiments Repeat this method for Experiments 2–5, using 10.0 cm3 of FB 4 and the volumes of water and FB 5 shown in Table 2.1. In each case, measure and record the maximum temperature. Carry out two further experiments, Experiments 6 and 7, which will enable you to determine more precisely the volume of FB 5 that gives the largest maximum temperature. Record your measurements in Table 2.1. Results initial temperature of FB 4 = .............................. °C Table 2.1 volume of water volume of FB 5 maximum experiment / cm3 / cm3 temperature / °C 1 9.00 1.00 2 7.00 3.00 3 5.00 5.00 4 3.00 7.00 I 5 1.00 9.00 II 6 III 7 IV [4] (ii) Use the intersection on your graph in (b)(i) to determine the volume of FB 5 required to neutralise 10.0 cm3 of FB 4. volume of FB 5 = .............................. cm3 [1] (c) Calculations (i) Calculate the amount, in mol, of hydrochloric acid in the volume of FB 5 in (b)(ii). (If you were unable to determine an answer to (b)(ii), use 5.10 cm3 as the volume of FB 5. This may not be the correct answer.) amount of HCl = .............................. mol Deduce the amount, in mol, of sodium hydroxide in 10.0 cm3 of FB 4. amount of NaOH = .............................. mol [1] (ii) Calculate the energy change, in J, when the amounts of reagents in (c)(i) neutralise each other. Show your working. energy change = .............................. J [1] (iii) Use your answer to (c)(ii) to calculate the enthalpy change, in kJ mol–1, when one mole of FB 4 is neutralised by one mole of FB 5. enthalpy change = .......... .............................. kJ mol–1 [1] sign value [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) l Temperature for all 7 volumes recorded. 4 ll All thermometer readings including initial temperature (if recorded) ending in .0 or .5 lll Selects two additional volumes of FB 5 for experiments 6 and 7 AND total volume for each additional experiment = 10.00 cm3 AND all four volumes recorded to 2 d.p. and ending in 0 or 5 lV Calculate the difference between the initial temperature of FB 4 and the temperature recorded in experiment 3 for both the supervisor and the candidate. Mark awarded if difference between candidate temperature rise in experiment 3 and the supervisor temperature rise is less than or equal to 1.0 °C. 2(b)(i) l Unambiguous labelled axes with (maximum) temperature on the y-axis and volume of FB 5 on 4 the x-axis OR correct units AND suitable linear scales. ll Points plotted accurately. lll two lines of best fit drawn (straight line or smooth curve) that account for all the plotted points ignoring any points labelled anomalous IV Both lines extrapolated to intersect. 2(b)(ii) Volume at intersection read correctly. 1 2(c)(i) Correctly calculates 1 2.00  volume from amount HCl = (b)(ii) / 1000 mol AND amount NaOH = amount HCl AND both answers to 2 – 4 sf 2(c)(ii) Correctly uses 1 Energy = 20  4.18  T J T = max T – initial T of FB 4 AND answer to 2 – 4 sf 2(c)(iii) Correctly uses 1 Enthalpy change = ((c)(ii) / 1000) / (c)(i) kJ mol−1 AND negative sign AND answer to minimum 2 sf

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3 (a) (i) FB 6, FB 7 and FB 8 are aqueous solutions of different compounds that each contain at least one oxygen atom. Carry out the following tests and record your observations in Table 3.1. Three of the tests have been done for you. Use a 1 cm depth of solution in a test‑tube for each test. Table 3.1 observations test FB 6 FB 7 FB 8 Test 1 Add a small spatula measure No change. No change. of manganese(IV) oxide. Test 2 Add a 1 cm length of magnesium. No change. Test 3 Add a few drops of aqueous iron(II) sulfate. [4] (ii) Use your observations in Table 3.1 to suggest a possible formula for each of FB 6, FB 7 and FB 8. FB 6 .............................. FB 7 .............................. FB 8 .............................. [2] (b) FB 9 contains two anions and two cations, three of which are listed in the Qualitative analysis notes. (i) To a small spatula measure of FB 9 in a test‑tube, add a 2 cm depth of dilute nitric acid. Record your observations. Keep the resulting solution for the test in (b)(ii). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) To the solution from (b)(i), add a few drops of aqueous silver nitrate. Then add excess aqueous ammonia. Record your observations. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Make an aqueous solution of FB 9 by adding a 5 cm depth of distilled water to a spatula measure of FB 9 in a test‑tube. Carry out the following tests on the aqueous solution of FB 9 and record your observations in Table 3.2. Table 3.2 test observations Test 1 To a 1 cm depth in a boiling tube, add aqueous sodium hydroxide, then warm. Test 2 To a 1 cm depth in a test‑tube, add a few drops of dilute hydrochloric acid, then add a few drops of aqueous chlorine. Empty and rinse the test‑tube with water immediately after use. [2] (iv) Use your observations in (b)(i), (b)(ii) and Table 3.2 to deduce the formulae of the cations and anions in FB 9. If you are unable to identify an ion, write ‘unknown’. cations .............................. and .............................. anions .............................. and .............................. [2] (v) Write an ionic equation for the reaction in (b)(ii). Include state symbols. ..................................................................................................................................... [1] [Total:14]

Mark scheme: 3(a) 4 observations test FB 6 FB 7 FB 8 Test 1 No change No change Fizzing * MnO2 Heat released * (Gas / oxygen) relights a glowing splint * Test 2 Fizzing * No change / no fizzing No change Mg Heat released (only award if not awarded for FB 8 test 1) (Gas / hydrogen ignites with) a pop (sound) with a lit splint * Test 3 No change * Green ppt * Either solution OR turns FeSO4 (award one * if both no AND yellow / yellow-orange / orange- changes highlighted correctly (ppt) turns brown brown / yellow-brown / (light) brown * recorded) (on standing) * Fizzing * (Gas / oxygen) relights a glowing splint (only award if not awarded in FB 8 test 1 above) 2 * = 1 mark (round down) 3(a)(ii) FB 6 CH3COOH / formula of any acid containing O 2 FB 7 NaOH / formula of any Group 1 hydroxide FB 8 H2O2 3(b)(i) Fizzing* 2 Forms colourless solution* Attempts limewater test* Gas gives white ppt with limewater * 3(b)(ii) Cream / off-white ppt 1 AND Partially soluble / insoluble in (aqueous) ammonia 3(b)(iii) M1 Test 1 1 No change / no ppt AND (on heating) gas / fizzing / ammonia turns litmus blue M2 Test 2 1 Brown or yellow solution OR turns brown or yellow 3(b)(iv) cations: NH4+ and unknown 2 anions: Br− and CO32− 3(b)(v) 1 Ag+(aq) + Br−(aq) → AgBr(s)

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