Cambridge A Level Chemistry 9701 — 2023 Oct/Nov Paper 3 · Variant 3

1 You will carry out a titration to determine the concentration of a solution of potassium manganate(VII), KMnO4. Potassium manganate(VII) reacts with excess acidified potassium iodide to produce iodine. This iodine is titrated with aqueous sodium thiosulfate. FA 1 is aqueous potassium manganate(VII), KMnO4. FA 2 is 0.500 mol dm–3 potassium iodide, KI. FA 3 is 0.120 mol dm–3 sodium thiosulfate, Na2S2O3. FA 4 is dilute sulfuric acid, H2SO4. FA 5 is starch indicator. (a) Method Titration ● Fill the burette with FA 3. ● Pipette 25.0 cm3 of FA 1 into a conical flask. ● Use the 25 cm3 measuring cylinder to add 10.0 cm3 of FA 2 to the conical flask. ● Use the 50 cm3 measuring cylinder to add 15.0 cm3 of FA 4 to the conical flask. ● Perform a rough titration by adding FA 3 from the burette to the conical flask until the solution is yellow. ● Then add several drops of FA 5. Continue the titration until the mixture in the flask becomes colourless. This is the end-point. ● Record the initial and final 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 FA 3 added in each accurate titration. Keep FA 2 and FA 5 for use in Question 3(b). 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 the mean value. The iodine produced by FA 1 required .................................... cm3 of FA 3. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to the appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium thiosulfate in the volume of FA 3 calculated in (b). amount of Na2S2O3 = .................................................. mol [1] (iii) The reaction by which iodine is produced is shown. 2KMnO4(aq) + 10KI(aq) + 8H2SO4(aq) 6K2SO4(aq) + 2MnSO4(aq) + 5I2(aq) + 8H2O(l) During the titration, sodium thiosulfate reacts with the iodine produced. 2Na2S2O3(aq) + I2(aq) 2NaI(aq) + Na2S4O6(aq) Calculate the concentration of potassium manganate(VII), in mol dm–3, in FA 1. concentration of KMnO4 = .......................................... mol dm–3 [2] (iv) A laboratory technician purchased a bottle containing 50.00 g of potassium manganate(VII), KMnO4, for this practical examination. Using your answer to (c)(iii), calculate the maximum volume of FA 1, in dm3, that the technician can prepare using the contents of this bottle. volume of FA 1 = .................................................. dm3 [1] (d) A student suggested that the accuracy of the experiment would be increased by using a 10 cm3 pipette to measure FA 2. State whether the student is correct. Explain your answer. ................................................................................................................................................... ............................................................................................................................................. [1]

Mark scheme: Question Answer Marks 1(a) I All the following data are recorded • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations II Appropriate headings and units shown in the accurate titration table and titre values recorded for accurate titrations. • initial / start and (burette) reading/volume • final / end and (burette) reading/volume • titre or volume / FA 3 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. The requirement to record to 0.05 applies to burette readings, including 0.00 cm3 (if this was the initial reading), but it does not apply to the titre. lV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. Do not include a reading if it is labelled ‘rough’. For assessment of accuracy (Q) marks, the Examiner should round all burette readings to the nearest 0.05 cm3. Check and correct subtractions. Then select the ‘best’ titres using the 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. These ‘best’ titres should be used to calculate the mean titre, expressed to nearest 0.01 cm3. Calculate the difference () between the candidate’s mean titre and the supervisor’s. Award three accuracy (Q) marks as shown below. Award V if  ⩽ 0.60 (cm3) 7 Award VI if  ⩽ 0.40 Award VII if  ⩽ 0.20 Tolerances for low titres: If supervisor’s titre is ⩽ 10.00 cm3, tolerances are 0.30, 0.20, 0.10 cm3. If supervisor’s titre is ⩽ 5.00 cm3 then tolerances are 0.15, 0.10, 0.05 cm3. 1(b) Correctly calculates 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 shown next to the two (or more) accurate readings selected. • The mean should be quoted to 2 dp, and be rounded correctly to nearest 0.01 cm3. (e.g. 26.625 cm3 must be rounded to 26.63 cm3) • Allow a mean expressed to 1 dp, if all accurate burette readings were given to 1 dp and the mean is exactly correct. (e.g. mean of 26.0 and 26.2 = 26.1 is allowed) (e.g. mean of 26.0 and 26.1 = 26.1 is wrong: it should be 26.05) 1(c)(i) All answers to parts (c)(ii), (c)(iii) and (c)(iv) quoted to 3 or 4 significant figures (sf). 1 1(c)(ii) Correct calculation of no of moles of Na2S2O3 1 No of moles = 0.120  mean titre/1000 1(c)(iii) Correct use of 1(c)(ii) 2 M1: amount of KMnO4 in 25 cm3 FA 1 = 0.2  (c)(ii) mol M2: Concn of KMnO4 = 40  0.2  (c)(ii) = 8  (c)(ii) mol dm–3 1(c)(iv) Correct use of (c)(iii) to calculate volume of FA 1 1 Volume of FA 1 = 50/158  1/(c)(iii) 1(d) No and FA 2 is used in excess (so exact volume is not important) 1

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2 You will determine the enthalpy change for the reaction of magnesium oxide with water to form magnesium hydroxide. MgO(s) + H2O(l) Mg(OH)2(s) The procedure will involve two experiments, one with magnesium oxide and the other with magnesium hydroxide. In each case you will react the solid with hydrochloric acid. FA 6 is magnesium oxide, MgO. FA 7 is 2.0 mol dm–3 hydrochloric acid, HCl. FA 8 is magnesium hydroxide, Mg(OH)2. (a) Experiment 1 is the determination of the enthalpy change of reaction, ΔH1, of magnesium oxide with hydrochloric acid. MgO(s) + 2HCl (aq) MgCl 2(aq) + H2O(l) (i) Method ● Support a cup in the 250 cm3 beaker. ● Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 7 into the cup. ● Measure and record the temperature of the solution in the cup. ● Weigh the container with FA 6. Record the mass. ● Tip all of FA 6 into the cup containing FA 7. FA 7 is in excess. ● Stir the mixture until the maximum temperature is obtained. Record the maximum temperature. ● Weigh the container with any residual FA 6. Record the mass. ● Calculate and record the mass of FA 6 used. ● Calculate and record the temperature rise. I II III [3] (ii) Calculate the energy released in your experiment. energy released = ...................................................... J [1] (iii) Calculate the enthalpy change of reaction, ΔH1, in kJ mol–1 of MgO(s), for the reaction of magnesium oxide with hydrochloric acid. Show your working. ΔH1 = ...... ............................. kJ mol–1 sign value [2] (b) Experiment 2 is the determination of the enthalpy change of the reaction, ΔH2, of magnesium hydroxide with hydrochloric acid. Mg(OH)2(s) + 2HCl (aq) MgCl 2(aq) + 2H2O(l) (i) Method ● Place the other cup in the beaker. ● Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 7 into the cup. ● Measure and record the temperature of the solution in the cup. ● Weigh the container with magnesium hydroxide, FA 8. Record the mass. ● Tip all of FA 8 into the cup containing FA 7. FA 8 is in excess. ● Stir the mixture until the maximum temperature is obtained. Record the maximum temperature. ● Weigh the container with any residual FA 8. Record the mass. ● Calculate and record the mass of FA 8 used. ● Calculate and record the temperature rise. I II III [3] (ii) Calculate the enthalpy change of reaction, ΔH2, in kJ mol–1 of Mg(OH)2(s), for the reaction of magnesium hydroxide with hydrochloric acid. Show your working. ΔH2 = ...... ............................. kJ mol–1 sign value [2] (c) Use your answers to (a)(iii) and (b)(ii) to calculate the enthalpy change, ΔHr, in kJ mol–1, for the reaction between magnesium oxide and water. The equation for the reaction is shown. MgO(s) + H2O(l) Mg(OH)2(s) Show your working. ΔHr = ...... ............................. kJ mol–1 sign value [1] [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. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a)(i) I Six unambiguous headings recorded, with correctly displayed units and in the space provided. 3 • Units: (oC), / g, in g, or oC or g against each value • (Mass of) container + FA 6 / MgO / contents / solid / g • (Mass of) (empty) container (+ residue) / g • (Mass of) FA 6 / MgO (used) / g • (Initial) temperature / oC • Maximum / final temperature / oC • Temperature rise / change / T / oC II Precision of readings in both experiments, 2(a) and 2(b). • Four weighings to same number of decimal places (2 or 3) • Four thermometer readings to .0 or .5 oC III Correct subtractions to calculate masses of FA 6 and FA 8 and for the temperature changes in both (a) and (b). 2(a)(ii) Correct calculation of energy change 1 Energy change = 30  4.18  temp rise (= 125.4  T) AND answer correctly rounded to 2–4 sf Do not penalise inappropriate sf twice in question 2. 2(a)(iii) Correct use of (a)(ii) 2 M1: Correct display Amount of MgO = mass used/40.3 mol (no answer required) M2: Correctly uses Enthalpy change = (a)(ii)/mol MgO  1/1000 kJ mol−1 and answer shows negative sign and is quoted to 2–4 sf 2(b)(i) I Six pieces of data recorded in the space provided (same readings are required as in (a)(i)) • Three temperatures • Three masses Accuracy marks in 2(b) • Round thermometer readings to 0.5 oC if necessary • Check supervisor’s and candidate’s subtractions for temp rise • Calculate difference () between supervisor and candidate (to 0.5 oC). II + III: Accuracy marks in 2(b) 3 Tsup / ºC <5.0 5.0–9.5 10.0–19.5 20.0–29.5 ⩾ 30.0 1 mark 1.0 1.5 2.0 2.5 3.0 2 marks 0.5 1.0 1.0 1.5 2.0 2(b)(ii) Correct use of data in enthalpy calculation 2 M1: Amount Mg(OH)2 reacting = 0.5  mol HCl (since Mg(OH)2 in excess) = 0.5  0.030  2.00 = 0.030 mol M2: Correct use of data in remainder of calculation (all three bullets correct) • Energy released = 30  4.18  temp rise in (b) (J) • Enthalpy change = heat released/1000 x 0.030 kJ mol−1 • Answer has negative sign and is quoted to 2–4 sig fig. 2(c) Enthalpy change (correctly calculated, with correct sign) 1 Hr = (a)(iii) – (b)(ii) kJ mol−1

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3 FA 9 contains one cation and one anion. The cation is one of calcium, magnesium, manganese(II) or zinc. The anion is oxide or carbonate. (a) Describe reactions to identify FA 9. You are advised to test for the anion first. State any conditions needed for these reactions. Carry out your reactions and record all your observations. Deduce the formula of FA 9. FA 9 is .................... . [4] (b) (i) FA 10 is a solution of a double salt. It contains two cations and one anion, all of which are listed in the Qualitative analysis notes. Use a 1 cm depth of this solution of FA 10 in a test-tube for Tests 1–3. Table 3.1 test observations Test 1 Add an equal volume of FA 2, aqueous potassium iodide, then add FA 5, aqueous starch. Test 2 Add aqueous barium chloride (or barium nitrate), then add dilute hydrochloric acid (or nitric acid). Test 3 Add aqueous silver nitrate, then add aqueous ammonia. Test 4 In a boiling tube, add aqueous sodium hydroxide to a 1 cm depth of solution of FA 10, then warm gently and carefully, then remove from heat and add one piece of aluminium foil to the mixture. [5] (ii) Identify the three ions in FA 10 by writing the formula of each ion. The three ions are .................... and .................... and .................... . [2] (iii) Identify the two tests in (b)(i) that involve redox reactions. Justify your answer for one of the tests. Tests .......... and .......... involve redox reactions. justification ........................................................................................................................ ........................................................................................................................................... ..................................................................................................................................... [2] (iv) Give an ionic equation for the reaction of one of the ions in FA 10 with sodium hydroxide in Test 4. Include state symbols. ..................................................................................................................................... [1] [Total: 14]

Mark scheme: 3(a) Test for anion 4 EITHER Add specified mineral acid * Fizzing / effervescence / bubbling occurs * (Bubble) gas/CO2 through limewater * White ppt formed * Colourless solution formed or vigorous (owtte) reaction * OR Heat the solid * (Bubble) gas / CO2 through limewater * White ppt formed * Solid goes yellow / yellow-green (when hot) * Solid becomes paler on cooling / goes white (on cooling) * Test for cation To a solution * made using acid * Add (aqueous) sodium hydroxide or ammonia * White ppt formed * Soluble in excess alkali * [Maximum mark for observations above = 3] FA 9 is ZnCO3 * (formula required) 2 * = 1 mark. [Maximum mark for 3(a) = 4] 3(b)(i) Observations: 11 * available = 5 marks rounding down. 5 Test 1 (KI) (Solution) turns dark yellow / brown / red-brown / orange-brown / yellow-brown * (With starch) (goes) dark blue / blue-black / black * (Ignore state) Test 2 (Ba2+) White precipitate * (With acid) precipitate insoluble / no change * Test 3 (AgNO3) No change / no precipitate / no reaction / solution remains colourless/pale yellow * (with NH3) brown / red-brown / orange-brown / rust precipitate * Test 4 (NaOH etc.) Brown/red-brown/orange-brown/rust precipitate * Ppt is insoluble in excess NaOH * (with heat) Gas / ammonia turns (damp red) litmus blue * (with Al) effervescence / fizzing / bubbling occurs * Gas/H2 ‘pops’ with lighted splint or gas / ammonia turns (damp red) litmus blue * 3(b)(ii) Ions are NH4+ and Fe3+ and SO42– 2 3(b)(iii) M1: Tests 1 and 4 involve redox (both must be correct) 2 M2: Any one strand of explanation correct (from those listed below) • Iodide ion is oxidised to iodine (or 2I− → I2 + 2e−) • Fe3+ is reduced to Fe2+ (or Fe3+ + e− → Fe2+) • Fe3+ is the oxidising agent and I− is the reducing agent • Fe3+ is reduced and I– is oxidised • Iodine changes from oxidation state -1 to 0 • Aluminium is oxidised to aluminium ions (or Al → Al3+ + 3e−) • Water / hydroxide ions is / are reduced to hydrogen • Al is the reducing agent and water / OH− ions ions is / are the oxidising agent • Al is oxidised and water / OH− ions is / are reduced 3(b)(iv) Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) 1 OR NH4+(aq) + OH–(aq) → NH3(g) + H2O(l) or (g)

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