Cambridge A Level Chemistry 9701 — 2021 May/June Paper 3 · Variant 4

1 You will carry out a titration to determine the concentration of a solution of potassium manganate(VII). You will react potassium manganate(VII) with excess acidified potassium iodide to produce iodine. You will then titrate the iodine with sodium thiosulfate. FB 1 is hydrated sodium thiosulfate, Na2S2O3•5H2O. FB 3 is aqueous potassium manganate(VII), KMnO4. FB 4 is 0.50 mol dm–3 potassium iodide, KI. FB 5 is dilute sulfuric acid, H2SO4. starch indicator (a) Method Preparing a solution of FB 1 ● Weigh the stoppered container of FB 1. Record the mass in the space below. ● Tip all the FB 1 into the beaker. ● Reweigh the container with its stopper. Record the mass. ● Calculate and record the mass of FB 1 used. ● Add approximately 100 cm3 of distilled water to the FB 1 in the beaker. ● Stir the mixture with a glass rod until all the FB 1 has dissolved. ● Transfer this solution into the 250 cm3 volumetric flask. ● Wash the beaker with distilled water and transfer the washings to the volumetric flask. ● Rinse the glass rod with distilled water and transfer the washings to the volumetric flask. ● Make the solution in the volumetric flask up to the mark using distilled water. ● Shake the flask thoroughly. ● This solution of sodium thiosulfate is FB 2. Label the flask FB 2. Titration ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Use the 25 cm3 measuring cylinder to add 15 cm3 of FB 5 to the conical flask. ● Use the same measuring cylinder to add 10 cm3 of FB 4 to the conical flask. ● Perform a rough titration by adding FB 2 from the burette to the conical flask until the solution is yellow. Then add several drops of starch indicator and continue the titration until the mixture in the flask becomes colourless. This is the end-point. 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 of your burette readings and the volume of FB 2 added in each accurate titration. I II III IV V VI VII VIII [8] (b) From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations. Show clearly how you obtained this value. The iodine produced by FB 3 required .............................. cm3 of FB 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of hydrated sodium thiosulfate, FB 1, that you weighed. moles of Na2S2O3•5H2O = .............................. mol [1] (iii) Calculate the number of moles of sodium thiosulfate in the volume of FB 2 calculated in (b). moles of Na2S2O3 = .............................. mol [1] (iv) 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) Use your answer to (c)(iii) to calculate the concentration of KMnO4, in mol dm–3, in FB 3. concentration of KMnO4 = .............................. mol dm–3 [1] (v) Calculate the mass of KMnO4 needed to prepare 1.00 dm3 of FB 3. Show your working. mass of KMnO4 = .............................. g [1] (d) (i) Solution FB 3 was actually prepared by dissolving 3.16 g of KMnO4 in 1.00 dm3 of solution. Show how you would use your answer to (c)(v) to calculate the overall percentage error in your experiment. [1] (ii) A student suggested that the percentage error in the experiment would be reduced by using a 10 cm3 pipette to measure FB 4. State whether the student is correct. Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 16]

Mark scheme: 1(a) I Unambiguous headings with data are recorded in the space provided • (mass of) container with FB 1 • (mass of) (empty) container • (mass of) FB 1 used correctly subtracted Unit must be shown. 1 lI 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 1 III Titre values recorded for accurate titrations, and Appropriate 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 1 IV All accurate burette readings recorded to 0.05 cm3 1 V The final accurate titre recorded is within 0.1(0) 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. Examiner selects the best mean titre. Apply 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 products of (mass × titre) for candidate and supervisor, as shown below, to one d.p,. Product = mean titre × mass of FB 1 used (cm3 g) If no weighings are discernible, assume mass of FB 1 = 6.40 g Calculate the difference (δ) between the candidate’s product and the supervisor’s product. Award the accuracy (Q) marks as shown below. Write and ring supervisor’s value next to the accurate titration table of each candidate, also candidate mean value (calculated by examiner) and difference between supervisor’s and candidate’s value, δ. Question Answer Marks 1(a) Award accuracy marks as follows: VI Award if δ ⩽ 6.0 cm3 g VII Award if δ ⩽.4.0 cm3 g VIII Award if δ ⩽ 2.0 cm3 g (3) 8 1(b) Correctly calculates mean accurate titre • 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 dp and be rounded to nearest 0.01 cm3. 1 1(c)(i) Sig fig mark All answers given for (c)(ii), (iii), (iv) and (v) must be to 3 or 4 sf 1 1(c)(ii) Correctly calculates No of moles of thio = mass weighed / 248.2 1 1(c)(iii) Correct use of (ii) Moles thio used = (c)(ii) × mean titre / 250 1 1(c)(iv) Correct use of (iii) Concentration of KMnO4 = 1000 / 25 × 2 / 5 × 1 / 2 × (c)(iii) mol dm–3 = (c)(iii) × 8 1 1(c)(v) Correct use of (iv) Mass of KMnO4 (g dm–3) = (c)(iv) × 158(.0) 1 1(d)(i) Correct expression % error = ([3.16 – (c)(v)] / 3.16) × 100 or ([(c)(v) – 3.16] / 3.16) × 100 1 1(d)(ii) Student is not correct since KI / FB 4 is used in excess 1

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2 You will determine the enthalpy change for the reaction of ammonia with hydrogen chloride. NH3(g) + HCl (g) → NH4Cl (s) The procedure will involve two experiments. FB 6 is 2.0 mol dm–3 aqueous ammonia, NH3. FB 7 is 3.0 mol dm–3 hydrochloric acid, HCl. FB 8 is ammonium chloride, NH4Cl. (a) Experiment 1: Determination of the enthalpy change of neutralisation of aqueous ammonia with hydrochloric acid NH3(aq) + HCl (aq) → NH4Cl (aq) (i) ● Support a cup in the beaker. ● Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FB 6 into the cup. ● Measure and record the temperature of the solution in the cup. ● Rinse the 25 cm3 measuring cylinder with water and then with a little FB 7. ● Use the 25 cm3 measuring cylinder to add 25.0 cm3 of FB 7 to the FB 6 in the cup. ● Stir the mixture. ● Measure and record the maximum temperature. ● Calculate and record the temperature rise. [2] (ii) Calculate the energy released in your experiment. (Assume that 4.2 J change the temperature of 1.0 cm3 of solution by 1.0 °C.) energy released = .............................. J [1] (iii) Calculate the enthalpy change of reaction, ∆H1, in kJ mol–1, for the neutralisation of NH3(aq) with HCl (aq). Show your working. ∆H1 = ...... ............................. kJ mol–1 sign value [2] (b) Experiment 2: Determination of the enthalpy change of solution of ammonium chloride NH4Cl (s) + aq → NH4Cl (aq) (i) ● Support a cup in the beaker. ● Rinse the 50 cm3 measuring cylinder with distilled water. ● Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of distilled water into the second cup. ● Measure and record the temperature of the water in the cup. ● Weigh the container with FB 8. Record the mass. ● Tip all of the FB 8 into the water in the cup. ● Stir until all FB 8 dissolves and record the minimum temperature observed. ● Calculate and record the temperature change. ● Weigh and record the mass of the container with any residual FB 8. ● Calculate and record the mass of FB 8 used. [3] (ii) Calculate the enthalpy change of solution, ∆H2, in kJ mol–1, for FB 8, ammonium chloride. (Assume that 4.2 J change the temperature of 1.0 cm3 of solution by 1.0 °C.) ∆H2 = ...... ............................. kJ mol–1 sign value [2] (c) The values for the enthalpy changes of solution of ammonia and hydrogen chloride are given. NH3(g) + aq → NH3(aq) ∆H = –30.5 kJ mol–1 HCl (g) + aq → HCl (aq) ∆H = –74.8 kJ mol–1 From your answers to (a)(iii), (b)(ii) and the data above, use Hess’ Law to calculate the enthalpy change, ∆Hr, in kJ mol–1, for the reaction below. NH3(g) + HCl (g) → NH4Cl (s) ∆Hr = ...... ............................. kJ mol–1 sign value [1] [Total: 11] Qualitative analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen ● the formation of any precipitate and its solubility in an excess of the reagent added ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted.

Mark scheme: 2(a)(i) M1: Unambiguous table/ list of data – 3 pieces of data shown • two thermometer readings recorded with clear headings and units • temperature rise shown correctly calculated Units must be ºC either with each value or as heading or in body of text: (º C), / ºC, in ºC, in degrees Celsius. M2: Accuracy mark Check and correct temperature rise for supervisor and candidate. Write supervisor’s temp rise on the script in a ring Calculate the difference δ between these temperature rises. Award this Q mark if δ ⩽ 1.0 °C 2 2(a)(ii) Correctly calculates energy released Energy = 55 × 4.2 × temp rise (J) and answer given to 2–4 sf 1 2(a)(iii) M1: Shows that HCl is in excess by calculating n(NH3) and n(HCl) (no. of moles ammonia) = 2 × 0.030 = 0.060 mol and (no. of moles of HCl) = 3 × 0.025 = 0.075 (so is in excess) M2: Correct use • ΔH = energy / (1000 × no. of moles NH3 used) (= q / 0.06) (ΔH = (a)(ii)/60) • Answer must have negative sign and be expressed to 2–4 sf 2 Question Answer Marks 2(b)(i) M1: Unambiguous table/ list of data – 6 pieces of data shown with units • (mass of) container + FB 8 / solid / NH4Cl • (mass of) container (empty or with residual FB 8) • (mass of) FB 8 / solid / NH4Cl or mass used • initial temperature / thermometer reading • final / lowest / minimum temperature / thermometer reading • temp change / decrease M2: Correct data collected (all bullets below are correct) • two thermometer readings are shown • both thermometer readings recorded to .0 or .5 • two balance readings are shown • both balance readings to the same number of dp • subtractions correct M3: Accuracy mark Check and correct temp decrease for supervisor and candidate. Write supervisor’s temperature decrease on the script in a ring. Calculate the difference δ between these temperature decreases. Award this Q mark if δ ⩽ 1.5 °C 3 2(b)(ii) Correct expressions • Energy = 30 × 4.2 × temperature decrease • Moles NH4Cl = mass used / 53.5 • Enthalpy change = energy / (1000 × moles NH4Cl) • Answer given with positive sign and 2 or more sig figs 2 bullets = 1 mark (round down) 2 2(c) Correctly uses data and answers (a)(iii) and (b)(ii) ΔH = – 30.5 – 74.8 + ans 2(a)(iii) – ans 2(b)(ii) or correct numerical answer given to 2 or more sf 1

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3 (a) FB 9 contains one anion and one cation both of which are listed in the Qualitative Analysis Notes. (i) Heat a small spatula measure of FB 9 strongly in a hard-glass test-tube. Allow the test-tube and contents to cool. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Add a small spatula measure of FB 9 to a 3 cm depth of dilute sulfuric acid in a test‑tube. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (iii) If necessary, pour off the solution obtained in (a)(ii) in order to separate it from any remaining solid. Divide this solution into two equal portions in boiling tubes. Carry out the following tests and record your observations. To the first boiling tube add aqueous sodium hydroxide. ............................................................................................................................................. ............................................................................................................................................. To the second boiling tube add aqueous ammonia. ............................................................................................................................................. ............................................................................................................................................. [2] (iv) Suggest the identity of FB 9. FB 9 is ...................................................... . [1] (b) FB 10 contains one anion and one cation. (i) Carry out the following tests and record your observations in the table. test observations Test 1 To a 1 cm depth of aqueous copper(II) nitrate in a boiling tube, add an equal volume of FB 10, then warm the mixture gently and carefully. Then add one piece of aluminium foil. Test 2 Warm a 1 cm depth of FB 10 gently in a boiling tube. Add one piece of aluminium foil. Allow the reaction to continue for one minute, then decant the solution into a boiling tube and add dilute hydrochloric acid until in excess. Test 3 To a 1 cm depth of aqueous chromium(III) sulfate in a test‑tube, add FB 10 dropwise. [5] (ii) Deduce the identity of the ions in FB 10. If you were unable to deduce the identity of an ion, write ‘unknown’. cation anion [1] [Total: 13] Qualitative analysis notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. pale blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Mark scheme: 3(a)(i) Observations when heated White solid / powder * Condensation in tube / water vapour formed / steam formed / droplets / liquid inside top of test-tube * Yellow (residue) formed (when hot) * (After cooling,) solid / residue becomes paler / goes white * 2 asterisks (*) = 1 mark (round down) 3(a)(ii) Observations with sulfuric acid (Mark as four ‘*’) fizzing / bubbling / effervescence * reaction is rapid / vigorous or colourless solution formed / solid / FB 9 dissolves / disappears * Attempts to test with limewater * Gas / CO2 turns lime water milky / cloudy white / white ppt * 2 asterisks (*) = 1 mark (round down) 2 3(a)(iii) Tests on solution (Mark as four asterisks) With NaOH: white ppt * soluble / forms (colourless) solution in excess (NaOH) * With NH3: white ppt * soluble / forms (colourless) solution in excess (NH3) * 2 asterisks (*) = 1 mark (round down) 2 3(a)(iv) FB 9 is zinc carbonate / ZnCO3 1 Question Answer Marks 3(b)(i) Observations with FB 10 Test 1 + Cu2+(aq): (pale) blue precipitate * then heat: black or dark brown precipitate / solid (formed) * + Al: fizzing / bubbling / effervescence * gas/NH3 (with Al) turns (red) litmus blue * Test 2 + Al: fizzing / bubbling / effervescence * (gas/hydrogen) ‘pops’ with a lighted spill/splint or gas has no effect on red litmus paper (OWTTE) * Al /solid dissolves/ disappears or colourless solution forms * then + HCl: (faint) white / pale grey / off white precipitate forms * Test 3 + Cr3+(aq): grey-green precipitate * then + excess FB 10: precipitate dissolves in excess (FB 10) or green solution formed * 2 asterisks (*) = 1 mark (round down) 5 3(b)(ii) Conclusion cation anion unknown OH−/ hydroxide 1

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