Cambridge A Level Chemistry 9701 — 2025 Oct/Nov Paper 3 · Variant 8

1 Many hydrated salts decompose when heated, losing water of crystallisation. The number of molecules of water of crystallisation, x, in hydrated aluminium sulfate can be determined by heating until it becomes anhydrous: x is an integer. Al 2(SO4)3•xH2O(s) Al 2(SO4)3(s) + xH2O(g) FB 1 is hydrated aluminium sulfate, Al 2(SO4)3•xH2O. (a) Method • Weigh the crucible with its lid. Record the mass. • Add between 1.80 and 2.00 g of FB 1 to the crucible. • Weigh the crucible, lid and FB 1. Record the mass. • Place the crucible on the pipeclay triangle. Gently heat the crucible and contents for approximately 2 minutes with the lid on. • Remove the lid. Heat the crucible and contents strongly for approximately 5 minutes. • Replace the lid and leave the crucible and residue to cool for at least 5 minutes. While the crucible is cooling, you should begin work on Questions 2 or 3. • Reweigh the crucible and contents with the lid on. Record the mass. • Remove the lid. Heat the crucible and contents strongly for a further 2 minutes. • Replace the lid and leave the crucible and residue to cool for at least 5 minutes. • Reweigh the crucible and residue with the lid on. Record the mass. • Calculate and record the mass of FB 1 used, the mass of residue obtained and the mass lost during heating. Results I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of water of crystallisation lost during the thermal decomposition of FB 1. amount of H2O lost = .............................. mol [1] (ii) Calculate the amount, in mol, of anhydrous residue produced by the thermal decomposition. Show your working. amount of Al 2(SO4)3 produced = .............................. mol [1] (iii) Calculate the number of molecules of water of crystallisation in the formula of hydrated aluminium sulfate, Al 2(SO4)3•xH2O. x = .............................. [1] (c) (i) State how the appearance of the residue compares with the appearance of the hydrated solid before heating. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Suggest why the crucible and contents are heated with the crucible lid on for the first two minutes of the experiment. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) A student carries out the experiment in (a), but obtains a value for x that is higher than expected. The student suggests that this could be because the hydrated aluminium sulfate is contaminated with some anhydrous aluminium sulfate. State whether the student’s suggestion is correct. Explain your answer. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 11]

Mark scheme: 1(a) I Seven unambiguous headings for readings, with correctly displayed units, in results space 1 • (mass of) (empty) crucible / g • (mass of) crucible + FB 1 / (hydrated) aluminium sulfate / g • (mass of) crucible and residue / contents, after 1st heating / g • (mass of) crucible and residue / contents, after 2nd heating / g • (mass of) FB 1 / g • (mass of) residue / g • (mass of) water / mass loss / g II Four weighings in space provided 1 • All four weighings recorded to same decimal places (either to two or to three). • Reading after 2nd heating is within +0.02 and −0.05 g of reading after 1st heating. III Correct subtractions to give masses of FB 1, residue and mass lost. 1 • All masses correctly subtracted. • Mass of FB 1 used in range 1.80 g–2.00 g (from weighings) IV and V: Accuracy (Q) marks in (a) 2 Calculate candidate’s mass ratio (to 2 d.p.) = mass FB 1 / mass of residue IV award if ratio is in the range 1.56–2.12 (inclusive) V award if ratio is in the range 1.66−2.02 (inclusive) 1(b)(i) Correct calculation, amount of water amount = mass loss / 18 mol AND answer to 2–4 sf 1 1(b)(ii) Correct calculation, amount of FB 1 1 amount = mass of residue / 342.3 mol AND answer to 2–4 sf 1(b)(iii) Correct use of mole ratio 1 x = (b)(i) / (b)(ii) AND answer given as closest integer Question Answer Marks 1(c)(i) FB 1 / solid at start / before heating is crystalline / finely divided 1 AND residue is lumpy / ‘crusty’ OR has a ‘skin’ 1(c)(ii) Lid is to prevent solid / solution from spitting / frothing out. 1 1(c)(iii) Student is not correct. 1 AND mass / amount of water / lost will be lower / too low / mass OR amount of anhydrous (salt) will be higher / too high OR ratio mol water / mol residue will be lower. Question Answer Marks

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2 The number of molecules of water of crystallisation, y, in hydrated iron(II) sulfate can be determined by titration with acidified potassium manganate(VII): y is an integer. FB 2 is aqueous iron(II) sulfate, containing 30.00 g dm–3 of FeSO4•yH2O. FB 3 is aqueous potassium manganate(VII), containing 3.48 g dm–3 of KMnO4. FB 4 is 1.0 mol dm–3 sulfuric acid, H2SO4. (a) Method • Fill the burette with FB 3. • Pipette 25.0 cm3 of FB 2 into a conical flask. • Use the 25 cm3 measuring cylinder to transfer approximately 10 cm3 of FB 4 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 below, all your burette readings and the volume of FB 3 added in each accurate titration. 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 2 required .............................. cm3 of FB 3. [1] (c) Calculations (i) Calculate the amount, in mol, of potassium manganate(VII) present in the volume of FB 3 in (b). Show your working. amount of KMnO4 = .............................. mol [2] (ii) An incomplete equation for the reaction of iron(II) ions with manganate(VII) ions is shown. The mole ratio of Fe2+ and MnO4 – is given correctly. Complete the equation. 5Fe2+(aq) + MnO4–(aq) + ........ H+(aq) ........ Fe3+(aq) + Mn2+(aq) + ........H2O(l) [1] (iii) Calculate the concentration of iron(II) sulfate, in mol dm–3, in FB 2. concentration of FeSO4 = .............................. mol dm–3 [1] (iv) Calculate the value of y in FeSO4•yH2O. y = .............................. [2] (d) A student suggests that the experiment is more accurate if FB 4 is measured with a pipette. State whether you agree with the student. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (e) Aqueous solutions of iron(II) sulfate are slowly oxidised by air. State what effect this oxidation would have on the value of y calculated in (c)(iv). Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 16] 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 are recorded 1 • two burette readings AND titre for the rough titration • initial and final burette readings for two (or more) accurate titrations II Titre values recorded for accurate titrations, AND 1 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 3 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 1 IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre 1 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 candidate’s mean titre value. Calculate the supervisor’s mean titre value. Calculate the difference () between the candidate’s mean titre and the supervisor’s mean titre. Award accuracy Q marks as follows: 3 V award if  ⩽ 0.50 cm3 VI award if  ⩽ 0.30 cm3 VII award if  ⩽ 0.20 cm3 Question Answer Marks 2(b) Correctly calculates 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. 2(c)(i) M1 Shows working 2 3.48 / 158 OR 3.48  (b) / 1000 OR (b) / 1000  1 / 158 M2 Correctly uses amount KMnO4 used = 3.48 / 158  (b) / 1000 AND final answer given to 3 or 4 sf 2(c)(ii) 5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) 1 2(c)(iii) Correct use 1 Concentration = 2(c)(i)  5  1000 / 25 mol dm–3 AND final answer given to 3 or 4 sf 2(c)(iv) Correct use 2 M1 Mr of hydrated iron(II) sulfate = 30.00 / (c)(iii) M2 y = [Mr – 151.9] / 18 AND answer is given as an integer 2(d) Student is incorrect 1 AND H2SO4 is used in excess (so the exact volume does not matter) / H2SO4 / the acid is not the limiting reagent 2(e) y will be greater AND one of: [1] • lower titre • fewer moles of KMnO4 • fewer moles / decreased concentration of Fe2+ / FeSO4 • higher Mr Question Answer Marks FB 5 is Zn; FB 6 is ZnSO4(aq); FB 7 is ZnCO3

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3 (a) (i) Use very small quantities of solid FB 5 and carry out each of the tests described in Table 3.1. Identify any gases produced. Table 3.1 test observations Test 1 Pour a 1 cm depth of aqueous iron(III) chloride into a test-tube. Add a small spatula measure of FB 5. Leave the test-tube to stand for about 3 minutes, then pour off some of the solution into another test-tube and add aqueous sodium hydroxide. Test 2 Pour a 1 cm depth of aqueous copper(II) sulfate into a test-tube. Add a small quantity of FB 5. Leave the test-tube to stand for about 3 minutes. Test 3 Pour a 1 cm depth of dilute sulfuric acid into a test-tube and add 2 drops of aqueous copper(II) sulfate. Then add a small quantity of FB 5. [4] (ii) FB 6 is the filtrate obtained after filtering the mixture that remains at the end of Test 3 in (a)(i). Add aqueous ammonia to FB 6. Record your observations. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Identify FB 5. FB 5 is ............................. . [1] (iv) Using your observations, explain why the reaction in Test 2 is a redox reaction. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Give the ionic equation for the first reaction observed in (a)(ii). Include state symbols. ..................................................................................................................................... [1] (b) FB 7 contains one anion and one cation. The anion contains oxygen but not nitrogen. Both ions are listed in the Qualitative analysis notes. (i) Transfer a small spatula measure of FB 7 into a hard-glass test-tube. Heat gently at the start, then strongly until no further change occurs. Leave the test-tube to cool. Record all your observations. Identify any gases produced. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (ii) Carry out one further positive test to confirm the identity of the anion in FB 7. Record only the results shown in a positive test. Describe the test you carry out and the observations you make in the space below. The anion in FB 7 is ............................. . [2] [Total: 13]

Mark scheme: 3(a)(i) 4 2 • = 1 mark Test 1 (FeCl3) • (Solution) becomes colourless / paler yellow / pale green • Fizzing / effervescence • (Pale) green / green-white precipitate (with NaOH) • Precipitate insoluble / no change with excess NaOH / ppt / solid turns brown (at surface) Test 2 (CuSO4) • Pink-brown AND precipitate / solid / residue • (Solution) gets paler (blue) OR (solution) turns colourless • gets hotter (or in Test 3) Test 3 (H2SO4) • Fizzing / effervescence • (Gas) pops with lighted splint • (Gas is) hydrogen 3(a)(ii) Ammonia gives a white precipitate AND soluble in excess 1 3(a)(iii) FB 5 is zinc / Zn 1 3(a)(iv) Copper is formed AND Copper ions gain electrons 1 OR Copper changes oxidation state from (+)2 to 0 / Cu2+ changes oxidation state to 0 OR zinc ions formed AND zinc loses electrons OR Zinc / Zn changes oxidation state from 0 to (+)2 3(a)(v) Zn2+(aq) + 2OH−(aq) → Zn(OH)2(s) 1 Question Answer Marks 3(b)(i) 3 2 • = 1 mark • (FB 7 is a) white powder / solid • Condensation / water droplets • (Solid / FB 7) turns yellow (or yellow-green) (when hot) • Residue / solid goes paler (on cooling) or residue is white • Attempts to test gas with limewater • (Gas) gives white precipitate with limewater • Gas is CO2 / carbon dioxide (evidence needed) 3(b)(ii) M1 Add any specified mineral acid (name OR correct formula) to FB 7. 2 M2 Fizzing / effervescence AND carbonate / CO32−

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