Cambridge A Level Chemistry 9701 — 2025 May/June Paper 3 · Variant 2

1 Acids donate H+ in aqueous solution. The number of moles of H+ donated per mole of acid is the proticity of the acid. For example, sulfuric acid is diprotic as it donates two moles of H+ per mole of acid. In this experiment you will carry out a titration to determine the proticity of citric acid, C6H8O7. FB 1 is aqueous citric acid containing 7.50 g dm–3 C6H8O7. FB 2 is aqueous sodium hydroxide containing 4.50 g dm–3 NaOH. FB 3 is thymolphthalein indicator. (a) Method • Fill the burette with FB 2. • Pipette 25.0 cm3 of FB 1 into a conical flask. • Add a few drops 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. 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) Calculate the amount, in mol, of citric acid in 25.0 cm3 of FB 1. Show your working. amount of C6H8O7 = .............................. mol [2] (ii) Calculate the amount, in mol, of sodium hydroxide in the volume of FB 2 in (b). amount of NaOH = .............................. mol [1] (iii) Citric acid is triprotic. Show whether your results in this experiment support this statement. [1] (iv) Complete the equation for the reaction of citric acid, C6H8O7, with sodium hydroxide. C6H8O7 + …… NaOH .............................. + .............................. . [1] (v) Citric acid does not contain a chiral carbon atom. Draw a possible structural formula of a molecule of citric acid. [1] (d) A student uses a pipette labelled 25.0 ± 0.06 cm3 to measure FB 1. The student suggests that it is more accurate to measure the volume of FB 1 with a burette instead of the pipette. State whether the student’s suggestion is correct. Explain your answer. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 15]

Mark scheme: Question Answer Marks 1(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 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 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 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: 3 V Award if  ⩽ 0.50 cm3 VI Award if  ⩽.0.30 cm3 VII Award if  ⩽ 0.20 cm3 1(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 dp and be rounded to nearest 0.01 cm3. (e.g. 26.675 cm3 must be rounded to 26.68 cm3) 1(c)(i) Correctly uses (answer to 3 or 4 sf) 2 M1: Amount citric acid in titration = 25 / 1000  7.50 / 192 M2: 9.766  10−4 mol OR 9.77  10−4 mol 1(c)(ii) Correctly calculates 1 Amount of NaOH used = (b) / 1000  4.5 / 40 mol AND answer to 3 or 4 sig fig 1(c)(iii) Calculated mole ratio = (c)(ii) / (c)(i) 1 OR stated mole ratio of 2.5 ⩽ mole ratio ⩽ 3.5 AND results support the statement / acid is triprotic. OR stated mole ratio ⩽ 2.5 or ⩾ 3.5 AND results do not support the statement / acid is not triprotic. 1(c)(iv) Equation 1 C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O 1(c)(v) Any valid displayed/structural/skeletal formula containing 3 COOH groups 1 Expected answer is HOOCCH2C(OH)COOHCH2COOH 1(d) Student is not correct 1 AND uncertainty in burette volume = 2  0.05 (which is greater than 0.06)

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2 You will determine the enthalpy change for the reaction of aqueous citric acid with aqueous sodium hydroxide to form aqueous sodium citrate. The procedure involves two experiments using solid citric acid. FB 4 is citric acid, C6H8O7. Use this for Experiment 1. FB 5 is citric acid, C6H8O7. Use this for Experiment 2. FB 6 is 2.00 mol dm–3 sodium hydroxide, NaOH. (a) Experiment 1 is the determination of the enthalpy change of solution, ΔH1, for citric acid. This is the enthalpy change when one mole of citric acid dissolves in water. C6H8O7(s) + aq C6H8O7(aq) ΔH1 Method • Support one of the cups in the 250 cm3 beaker. • Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of distilled water into the cup. • Measure the temperature of the water in the cup. Record this temperature in the space for results. • Weigh the container with FB 4. Record the mass. • Tip all of the FB 4 into the water in the cup. • Stir the mixture until the minimum temperature is obtained. Record this temperature. • Weigh the container with any residual FB 4. Record the mass. • Calculate and record the mass of FB 4 used. • Calculate and record the temperature change. Results I II III [3] (b) Calculations (i) Calculate the energy change, in J, in your experiment. energy change = .............................. J [1] (ii) Calculate the enthalpy change of solution, ΔH1, in kJ mol–1, for dissolving 1.00 mol of solid citric acid, C6H8O7, in water. Show your working. ΔH1 = ...... ..................... kJ mol–1 [2] sign value (c) Experiment 2 is the determination of the enthalpy change, ΔH2, for the reaction of one mole of solid citric acid with aqueous sodium hydroxide. In this experiment, aqueous sodium hydroxide, FB 6, is used in excess. Method • Support the second cup in the beaker. • Add 4.80–5.00 g of FB 5 to the cup. Record your weighings. • Measure and record the temperature of FB 6 in its container. • Use the 50 cm3 measuring cylinder to transfer 50.0 cm3 of FB 6 into the cup with FB 5. • Stir the mixture until the maximum temperature is obtained. Record the maximum temperature. • Calculate and record the mass of FB 5 used. • Calculate and record the temperature change. Results I II III [3] (d) Calculations Calculate the enthalpy change of reaction, ΔH2, in kJ mol–1, for the reaction of 1.00 mol of solid citric acid, C6H8O7, with aqueous sodium hydroxide. Show your working. ΔH2 = ...... ..................... kJ mol–1 [2] sign value (e) Use your values for ΔH1 and ΔH2 to calculate the enthalpy change, ΔHr , in kJ mol–1, for the reaction of 1.00 mol of aqueous citric acid with aqueous sodium hydroxide. ΔHr = ...... ..................... 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) M1 Unambiguous headings for mass, with units, correctly displayed, in (a) AND (c) 3 in (a): • (Mass of) container + FB 4 / g • (Mass of) container empty (or, with residue) / g • (Mass of) FB 4 (used / added) / g AND value for mass of FB 5 used in (c) is between 4.80 and 5.00 g M2 Unambiguous headings for temperature, with units, correctly displayed, in (a) AND (c) in (a) • Initial temperature (or temperature of water) / °C • Final / minimum temperature / °C • Temperature change / fall / T / °C in (c) • Initial temperature (or temperature of FB 6) / °C • Final / maximum temperature / °C • Temperature change / rise / T / °C M3 Precision of readings in (a) AND (c) • Both balance readings in (a) given to the same number of decimal places (2 or 3). • Four thermometer readings recorded to .0 or .5 °C 2(b)(i) Correctly calculates 1 Energy change = 30  4.18  temp decrease J AND answer correctly rounded to 2 – 4 sf 2(b)(ii) Correct use of (b)(i) 2 M1: Correct display Amount of citric acid = mass used / 192 mol (no answer required) M2: Correctly uses Enthalpy change = (b)(i) / mol citric acid  1 / 1000 kJ mol−1 AND answer shows positive sign AND is quoted to 2 – 4 sf 2(c) M1 Correct subtractions in (a) AND (c) 1 • Mass of FB 4 and FB 5 • Two temperature changes (ignore sign) Accuracy (Q) marks Round thermometer readings to 0.5 °C if necessary before calculating temperature rise. If the candidate has a temp. increase in (a) OR a temp. decrease in (c), award zero Q marks. If temperatures are clearly a factor of 10 too small, multiply by 10 before assigning marks II and III. Calculate the candidate’s mass ratio (to 2dp) = temp rise in (c) / mass of FB5 used Award accuracy (Q) marks as follows: 2 Award II if 2.75 ⩽ ratio ⩽ 3.75 Award III if 3.00 ⩽ ratio ⩽ 3.50 2(d) Correct use of data in enthalpy calculation 2 M1 Energy change = 50  4.18  temp rise in (c) (J) M2 Correct use of data in remainder of calculation (all three bullets correct) mass of FB • n = 5 / 192 mol energy • H2 = change / n  1 / 1000 kJ mol−1 • Sign is negative AND answer given to 2–4 sig fig 2(e) Enthalpy change (with correct sign) 1 Hr = (d) – (b)(ii) = H2 – H1

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3 (a) Solutions FB 7 and FB 8 each contain one cation and one anion. All of the ions are listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Table 3.1. Use a 1 cm depth of FB 7 or FB 8 in a test-tube for each test. Table 3.1 observations test FB 7 FB 8 Test 1 Add an equal volume of aqueous potassium iodide, then add excess aqueous sodium thiosulfate. Test 2 Add a small spatula measure of zinc powder. Leave the mixture to stand. Test 3 Add a few drops of aqueous silver nitrate. Test 4 Add aqueous ammonia. [6] (ii) Deduce the formula of FB 8. FB 8 is .......................... . [1] (iii) For FB 8, state the numbers of all the tests from Table 3.1 that involve a redox reaction. ..................................................................................................................................... [1] (iv) Give the ionic equation for one of the reactions in Test 4. Include state symbols. ..................................................................................................................................... [1] (b) (i) Carry out tests to identify the anion in FB 7. The anion does not contain sulfur. Record your tests and observations in a suitable form in the space below. You must use a boiling tube if any liquid is heated. [3] (ii) Deduce the formula of FB 7. FB 7 is ……………… . [1] [Total: 13]

Mark scheme: 3(a) 2 * = 1 mark (round down). Max. 6 marks 6 2  highlighted boxes = 1 * test observations FB 7 FB 8 Test 1 (5*) Brown colour Brown / red-brown solution (forms) * Add an equal volume (ignore state) * of potassium iodide (aq), then white ppt (formed)* turns colourless / goes (pale) yellow / pale add excess pink* sodium thiosulfate (aq). ppt soluble in excess / solution turns colourless* Test 2 (4*) Pink / brown / black solid* Fizzing / effervescence * Add a small spatula measure of zinc solution turns paler blue / colourless * (gas) pops with lighted splint * powder. Leave the mixture to stand. Test 3 (1*) no change / no ppt / no reaction AND ppt forms * Add a few drops of silver nitrate (aq). Test 4 (3*) (pale) blue ppt * brown / red-brown / rust ppt Add ammonia (aq). soluble in excess * AND insoluble in deep / dark blue solution formed / in excess (NH3) * excess * 3(a)(ii) FB 8 is FeCl3 1 3(a)(iii) Tests 1 AND 2 (are redox) 1 3(a)(iv) One equation from: 1 • Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) • Cu2+(aq/s) + 4NH3(aq) → [Cu(NH3)4]2+(aq) • Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) 3(b)(i) M1 Add (excess) NaOH to FB 7 AND heat AND add aluminium 3 M2 Gas / ammonia / fizzing turns (damp red) litmus blue M3 Add (acidified) KMnO4 AND remains purple / no change / not decolorised OR Add specified (dilute) mineral acid AND no change / no fizzing / no brown gas 3(b)(ii) FB 7 is Cu(NO3)2 1

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