Cambridge A Level Chemistry 9701 — 2025 Feb/March Paper 3 · Variant 3

1 Potassium alum is a hydrated salt containing aluminium ions, potassium ions and sulfate ions. 1 mol of hydrated potassium alum contains 12 mol of water of crystallisation. Hydrated potassium alum decomposes when heated, losing its water of crystallisation and becoming anhydrous. You will determine the formula of potassium alum by heating the hydrated salt until it becomes anhydrous. FA 1 is hydrated potassium alum. (a) Method • Weigh a crucible with its lid. Record the mass in the space for results. • Add all of the FA 1 to the crucible. • Weigh the crucible and lid with FA 1. Record the mass. • Calculate and record the mass of FA 1 added. • Place the crucible on the pipe-clay triangle. Heat the crucible and contents gently 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 may begin work on Question 2 or Question 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 residue obtained. 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 FA 1. amount of H2O lost = .............................. mol [1] (ii) Use the information given and your answer to (b)(i) to determine the amount, in mol, of potassium alum used. amount of potassium alum = .............................. mol [1] (iii) Calculate the relative formula mass, Mr, of anhydrous potassium alum. Mr = .............................. [1] (iv) Anhydrous potassium alum contains aluminium ions, potassium ions and sulfate ions. 1 mol of potassium alum also contains 1 mol of aluminium ions. Use the Mr you have calculated in (b)(iii) to suggest the formula of anhydrous potassium alum. Show your working. The formula = .............................. . [1] (c) (i) The uncertainty in a single balance reading for a two decimal place balance is 0.01 g. Calculate the maximum percentage error in your measurement of the mass of the residue of anhydrous potassium alum. Show your working. maximum percentage error = ..............................% [1] (ii) A student obtains a higher value for the relative formula mass, Mr, of anhydrous potassium alum than expected. The student incorrectly suggests that this is because some of the anhydrous potassium alum residue decomposes to aluminium oxide and potassium oxide during strong heating. Explain why the student’s suggestion is not correct. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 11]

Mark scheme: Question Answer Marks 1(a) I Six unambiguous headings for balance readings AND calculated values AND correct units 5 • (mass of) crucible (and lid) • (mass of) crucible (and lid) and FA 1 • (mass of) crucible (and lid) and residue / contents after first heating • (mass of) crucible (and lid) and residue / contents after second heating • (mass of) FA 1 • (mass of) residue • Units: / g, (g), in g (or g with every entry) II Recording of all weighings • All four weighings recorded to same number of decimal places (two or three). • Fourth reading is within +0.02 and –0.05 g of third reading. III Correct calculation of masses Correct masses of FA 1 and residue AND both answers to min 2 sf. Accuracy marks Calculate candidate’s mass ratio (to 2 d.p.) = mass FA 1 / mass of residue. IV award if ratio is within the range 1.70–2.00 (inclusive) V award if ratio is within the range 1.80–1.90 (inclusive) 1(b)(i) Correct calculation of amount of water 1 amount of H2O = (mass of FA 1 – mass of residue) / 18 AND answer to 2–4 s.f. 1(b)(ii) Correct use of amount of water 1 amount of potassium alum, FA 1 = (b)(i) / 12 AND answer to 2–4 s.f. 1(b)(iii) Correct use of amount of anhydrous potassium alum 1 Mr of anhydrous alum = mass of residue / (b)(ii) AND answer to 2–4 s.f. 1(b)(iv) Formula is Al K(SO4)2 1 AND (correct) working which shows use of (b)(iii) AND use of 27(.0) or 258.3. 1(c)(i) Correct expression 1 (Uncertainty U (for 2 d.p. balance) = 0.01) % error in weighing FA 1 = 2  U / mass of residue  100 AND answer given. 1(c)(ii) If anhydrous potassium alum decomposes: 1 mass loss will be higher / too high OR no longer is only water lost OR (calculated) moles of water / (b)(i) will be higher AND (calculated) amount / moles of residue / (anhydrous) potassium alum / (b)(ii) will be higher / too high. (so Mr of residue / (b)(iii) will be too low).

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2 Many oxidising agents are able to oxidise acidified potassium iodide to iodine in acidic conditions. The amount of iodine produced can be determined by titrating it with aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) 2NaI(aq) + Na2S4O6(aq) You will determine the change in oxidation state of an oxidising agent when it reacts with iodide ions. FA 2 is aqueous sodium thiosulfate, containing 22.00 g Na2S2O3•5H2O (Mr = 248.2) in 1.00 dm3. FA 3 is a 0.0175 mol dm–3 solution of an oxidising agent. FA 4 is 0.50 mol dm–3 potassium iodide, KI. FA 5 is 1.00 mol dm–3 sulfuric acid, H2SO4. FA 6 is starch solution. (a) Method • Fill the burette with FA 2. • Pipette 25.0 cm3 of FA 3 into a conical flask. • Use the 10.0 cm3 measuring cylinder to add 10 cm3 of FA 4, an excess, to the conical flask. • Use the 25.0 cm3 measuring cylinder to add 20 cm3 of FA 5, an excess, to the conical flask. • Add FA 2 from the burette until the solution becomes yellow. • Add about 10 drops of FA 6 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 FA 2 added in each accurate titration. Keep FA 3, FA 4, FA 5 and FA 6 for use in Question 3. 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 3 required .............................. cm3 of FA 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 sodium thiosulfate in the volume of FA 2 in (b). amount of Na2S2O3 = .............................. mol [1] (iii) Calculate the amount, in mol, of iodine that reacts with the amount of sodium thiosulfate in (c)(ii). amount of I2 = .............................. mol [1] (iv) Calculate the amount, in mol, of FA 3 used to produce the amount of iodine in (c)(iii). amount of FA 3 = .............................. mol [1] (v) Calculate the amount, in mol, of iodine produced by the reaction of 1 mol of FA 3 with potassium iodide. Give your answer to one decimal place. amount of I2 = .............................. mol [1] (vi) The oxidising agent in FA 3 is a compound of a transition metal, M. The redox reaction of FA 3 with iodide ions produces M2+ ions. Use your answer to (c)(v) to calculate the change in the oxidation state of M during this reaction. Show your working. The oxidation state of M changes from .............................. to .............................. . [2] (d) A student suggests that the experiment in (a) would be more accurate if the FA 5, sulfuric acid, is measured using a pipette. State whether the student is correct. 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 All the following data are recorded: 7 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 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 FA 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 marks Round burette readings to the nearest 0.05 cm3 then 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. The ‘best’ titres should be used to calculate the mean titre, expressed to the nearest 0.01 cm3. Calculate the supervisor’s mean titre to 2 decimal places. Calculate the candidate’s mean titre to 2 decimal places. Calculate the difference  between the candidate’s titre and the supervisor’s titre. V Award if  ⩽ 0.60 cm3 VI Award if  ⩽ 0.40 cm3 VII Award if  ⩽ 0.20 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 should be quoted to 2 d.p. and be rounded to nearest 0.01 cm3. 2(c)(i) answers to parts (c)(ii), (c)(iii) and (c)(iv) are given to 3 or 4 significant figures. 1 2(c)(ii) Correct calculation of amount of sodium thiosulfate 1 amount of sodium thiosulfate used = 22.0 / 248.2  titre / 1000 2(c)(iii) Correct use of (c)(ii) 1 amount I2 = 0.5  (c)(ii) 2(c)(iv) Correct calculation of amount of FA 3 1 amount FA 3 = 0.0175  25 / 1000 = 0.0004375 2(c)(v) Correct use of (c)(iii) and (c)(iv) 1 amount I2 = (c)(iii) / (c)(iv) AND answer to 1 decimal place. 2(c)(vi) M1 Working to show increase in oxidation state for I (during reaction) 2 = (c)(v)  (+)2 M2 M changes from (2 + M1) to (+) 2 AND both values are integers 2(d) Student is incorrect 1 AND FA 5 / sulfuric acid is used in excess (so the exact volume does not matter).

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3 (a) Carry out the following tests using FA 3 and record your observations in Table 3.1. Use a 1 cm depth of FA 3 in a test-tube for each test. Table 3.1 test observations Test 1 Add an equal volume of aqueous sodium hydroxide, then add an equal volume of aqueous sodium sulfite. Shake the mixture in the tube, then add sulfuric acid, FA 5. Test 2 Add an equal volume of sulfuric acid, FA 5, then add a small spatula measure of zinc. Allow the mixture to stand. Test 3 Add an equal volume of aqueous hydrogen peroxide. Give the formula of the gas formed in Test 3. The gas formed is .............................. . [4] (b) (i) FA 7 is a solution containing four ions, three of which are listed in the Qualitative analysis notes. Carry out the following tests and record your observations in Table 3.2. Use a 1 cm depth of FA 7 for each test. A boiling tube must be used for Test 1 and a test-tube for the other tests. Table 3.2 test observations Test 1 Add 0.5 cm depth of aqueous sodium hydroxide to a boiling tube, then warm the mixture carefully, then add aluminium foil. Test 2 Add several drops of aqueous potassium iodide, FA 4, then add a few drops of starch solution, FA 6. Test 3 Add the pieces of magnesium. Test 4 Add a few drops of aqueous barium nitrate or aqueous barium chloride, then add hydrochloric acid. Test 5 Add a few drops of aqueous silver nitrate, then add aqueous ammonia. [6] (ii) Give the formula of each of the four ions in FA 7. The ions are ........................ , ........................ , ........................ and ........................ . [2] (iii) Give the ionic equation for one reaction that takes place in Test 1 or Test 3 of (b)(i). Include state symbols. ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 3(a) Test 1 4 • (with NaOH) no change OR (solution) stays / remains purple • (with Na2SO3) green solution (formed at first) / (solution) turns green • (with Na2SO3) brown ppt / solid • (with acid) turned / formed AND colourless (solution). Test 2 • Purple turns colourless OR colourless solution formed • Fizzing / effervescence Test 3 • fizzing / effervescence • (dark) brown solid / ppt forms • (gas) re-lights glowing spill • gas is O2 Two points needed for each mark, to a maximum of 4 marks. 3(b)(i) Test 1 6 • (with NaOH) brown / red-brown / rust precipitate • (with NaOH) insoluble in excess (NaOH) • (on warming) gas / NH3 turns (red) litmus blue • (with Al) fizzing / effervescence Test 2 • (with KI) (solution) turns darker yellow / yellow-brown / orange-brown / red-brown / brown (with starch) turns dark blue / blue-black / black Test 3 • fizzing / effervescence (gas / H2) gives pop with lighted splint Test 4 • (with Ba2+) white precipitate • (with HCl) ppt remains / insoluble • (with HCl) yellow solution formed Test 5 • (with AgNO3) no change / no precipitate • (with NH3) brown / red-brown / rust precipitate • (with NH3) insoluble in excess Two points needed for each mark, to a maximum of 6 marks. 3(b)(ii) Correct identification of ions 2 H+, NH4+, Fe3+, SO42– 4 ions correct = 2 marks 2 or 3 ions correct = 1 mark 3(b)(iii) One correct ionic equation: 1 Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s) NH4+(aq) + OH–(aq) → NH3(g) + H2O(l or g) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) Mg(s) + 2Fe3(aq) → 2Fe2+(aq) + Mg2+(aq)

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