Cambridge A Level Chemistry 9701 — 2022 Oct/Nov Paper 3 · Variant 6

1 In this experiment you will determine the concentration of dilute hydrochloric acid by further diluting it and then titrating with aqueous potassium carbonate. FB 1 is dilute hydrochloric acid, HCl. This acid will also be used in Question 2. FB 2 is 8.46 g dm–3 anhydrous potassium carbonate, K2CO3. FB 3 is bromophenol blue indicator. (a) Method ● Use the 10 cm3 pipette to transfer 10.0 cm3 of FB 1 into the volumetric flask. ● Make this solution up to 250 cm3 using distilled water. ● Shake the volumetric flask and its contents thoroughly. Label this solution FB 4. ● Fill the burette with FB 4. ● Pipette 25.0 cm3 of FB 2 into a conical flask. ● Add a few drops of FB 3. ● 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 4 added I in each accurate titration. 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 4. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iv) and (c)(v) to the appropriate number of significant figures. [1] (ii) Use information on page 2 to calculate the amount, in mol, of potassium carbonate present in 25.0 cm3 of FB 2. amount of K2CO3 = .............................. mol [1] (iii) Give the ionic equation for the reaction taking place in the titration in (a). Include state symbols. .....CO32–...... + .....H+...... → .............................. + .............................. [1] (iv) Calculate the concentration, in mol dm–3, of HCl in FB 4. concentration of HCl in FB 4 = .............................. mol dm–3 [1] (v) Calculate the concentration, in mol dm–3, of HCl in FB 1. concentration of HCl in FB 1 = .............................. mol dm–3 [1] (d) A student uses a solution containing 8.46 g dm–3 of hydrated potassium carbonate in the titration in (a) instead of FB 2. State whether the student’s titre would be larger or smaller than your titre. Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (e) A student carries out the titration described in (a). By mistake, a 25 cm3 pipette is used to transfer 25.0 cm3 of FB 1 into a volumetric flask. The solution is made up to 250 cm3 with distilled water and labelled FB 4. State how the subsequent experimental procedure could be modified so that the titre obtained in (b) is not altered by this mistake. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 15]

Mark scheme: Question Answer Marks 1(a) I the following data must be shown 7 • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations II titre values shown 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 / FB 4 and 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 are to the nearest 0.05 cm3. IV the final accurate titre recorded is within 0.10 cm3 of any other accurate titre. V, VI, VII award V if d ⩽ 0.60 (cm3) award VI if d ⩽ 0.40 (cm3) award VII if d ⩽ 0.20 (cm3) where d is the difference between the supervisor’s and candidate’s mean titres 1(b) candidate calculates mean correctly to 2 decimal places (dp) 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 the nearest 0.01 cm3. 1(c)(i) all answers to (c)(ii), (c)(iv) and (c)(v) expressed to 3 or 4 significant figures (sf) 1 1(c)(ii) correctly calculates amount of K2CO3 in 25 cm3 1 8.46 1 −3 amount =  = 1.53 ( 0 )  10 (mol) 40 138.2 1(c)(iii) CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l) 1 1(c)(iv) correct use of (c)(ii) to calculate HCl concentration in FB 4 1 1000 concentration of HCl in FB 4 = 0.00153 ( 0 )  2  (mol dm−3) titre 1(c)(v) correct use of (c)(iv) to calculate HCl concentration in FB 1 1 250 concentration of HCl in FB 1 = (c)(iv)  (mol dm−3) 10 1(d) titre will be smaller / reduced 1 because amount / concentration of K2CO3 (in hydrated salt) is smaller 1(e) one of: 1 • use 62.5 cm3 / 2.5  the volume of FB 2 • change the concentration of FB 2 to 21.15 g dm−3 • make a subsequent dilution of FB 4 of 100 cm3 to 250 cm3

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2 In this experiment you will determine the enthalpy change, ∆H, for the dehydration of hydrated sodium carbonate to anhydrous sodium carbonate. Na2CO3•10H2O(s) → Na2CO3(s) + 10H2O(l) You will determine the enthalpy changes for the reactions of anhydrous sodium carbonate and hydrated sodium carbonate with excess hydrochloric acid. Then you will use Hess’s Law to calculate the enthalpy change for the reaction above. FB 1 is dilute hydrochloric acid, HCl. FB 5 is anhydrous sodium carbonate, Na2CO3. FB 6 is hydrated sodium carbonate, Na2CO3•10H2O. (a) Determination of the enthalpy change for the reaction of anhydrous sodium carbonate, FB 5, with excess hydrochloric acid. Method ● Support a cup in the 250 cm3 beaker. ● Use the measuring cylinder to transfer 30.0 cm3 of FB 1 into the cup. ● Measure and record the initial temperature of the acid. ● Weigh the container with FB 5. Record the mass. ● Slowly add all FB 5 to the acid in the cup. Note that the reaction will be vigorous. Avoid inhaling any acid spray produced. ● Stir until the maximum temperature is reached. Measure and record this temperature. ● Weigh the container with any residual FB 5. Record the mass. ● Calculate and record the mass of FB 5 used. ● Calculate and record the temperature change. Results I II III IV [4] (b) Calculations (i) Calculate the energy change during this reaction. energy change = .............................. J [1] (ii) Calculate the amount, in mol, of anhydrous sodium carbonate, FB 5, used. Show your working. amount of Na2CO3 = .............................. mol [1] (iii) Calculate the enthalpy change, in kJ mol–1, for the reaction between FB 5 and FB 1, shown below. Na2CO3(s) + 2HCl (aq) → 2NaCl (aq) + CO2(g) + H2O(l) enthalpy change for Na2CO3 = ...... ............................. kJ mol–1 sign value [1] (c) A student alters the method for the experiment in (a) and uses FB 1 at half its original concentration but doubles the volume used. The student suggests that the temperature change measured using their method would be more accurate than using the method in (a). State if you agree with the student. Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (d) Determination of the enthalpy change for the reaction of hydrated sodium carbonate, FB 6, with excess hydrochloric acid, FB 1. Method ● Support the second cup in the 250 cm3 beaker. ● Use the measuring cylinder to transfer 30.0 cm3 of FB 1 into the cup. ● Measure and record the initial temperature of the acid. ● Weigh the container with FB 6. Record the mass. ● Slowly add all the FB 6 to the acid in the cup. Note that the reaction will be vigorous. Avoid inhaling any acid spray produced. ● Stir until the minimum temperature is reached. Measure and record this temperature. ● Weigh the container with any residual FB 6. Record the mass. ● Calculate and record the mass of FB 6 used. ● Calculate and record the temperature change. Results I II III [3] (e) Calculations (i) Calculate the enthalpy change, in kJ mol–1, for the reaction. Na2CO3•10H2O(s) + 2HCl (aq) → 2NaCl (aq) + CO2(g) + 11H2O(l) enthalpy change for Na2CO3•10H2O = ...... ............................. kJ mol–1 sign value [2] (ii) Use the enthalpy changes calculated in (b)(iii) and (e)(i) to calculate the enthalpy change, in kJ mol–1, for dehydration of hydrated sodium carbonate. Na2CO3•10H2O(s) → Na2CO3(s) + 10H2O(l) enthalpy change for dehydration = ...... ............................. kJ mol–1 sign value [1] [Total: 14] 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 six pieces of data recorded 4 two balance readings, two thermometer readings, mass used, temperature rise and all shown in the Results space provided II correct subtractions in (a) to calculate the mass of FB 5 and the temperature change III, IV award III for d ⩽ 0.60 (°C g–1) award IV for d ⩽ 0.30 (°C g–1) rise in temperature where d is the difference between the supervisor’s and candidate’s ratio of calculated correctly to 2 dp mass of FB5 2(b)(i) correct calculation of energy change 1 energy change = 30  4.18  temperature rise (J) and answer given to 2 – 4 sf 2(b)(ii) correctly calculates amount of Na2CO3 1 mass used amount = (mol) 106 and answer given to 2 – 4 sf 2(b)(iii) correct use of (c)(i) and (c)(ii) 1 ( b )( i ) 1 enthalpy change =  (kJ mol−1) ( b )( ii ) 1000 and answer with negative sign and given to 2 – 4 sf 2(c) student is not correct because temperature rise will be halved / lower so greater percentage error in measurement 1 OR student is correct because there will be less spray / frothing out of the cup 2(d) I six unambiguous headings with correct units given for all readings (/ °C and / g, etc.) 3 • initial thermometer reading / temperature • minimum / final thermometer reading / temperature • (mass of) container and FB 6 / solid • (mass of) container (empty or with residual FB 6) • (mass of) FB 6 / solid used • temperature change / decrease II four readings recorded • two thermometer readings and two balance readings recorded in the space for Results • mass used is between 7.00 – 11.00 g • thermometer readings show a decrease in temperature of at least half of the supervisor’s value III precision of all readings in (a) and (d) • four balance readings recorded to same dp (two or more) • four thermometer readings recorded to .0 or .5°C 2(e)(i) worrect working shown 2 • energy change = 30  4.18  temperature decrease (J) mass used • amount of carbonate = (mol) 286 energy change 1 • enthalpy change =  (kJ mol−1) no. of moles 1000 • answer shown with positive sign and to 2 or more sf 2(e)(ii) correct calculation: 1 enthalpy of dehydration = 2(e)(i) – 2(b)(iii) (kJ mol−1)

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3 (a) FB 7 is an aqueous solution containing two anions listed in the Qualitative analysis notes. Neither of the anions contains nitrogen. (i) To a 1 cm depth of FB 7 in a test-tube add a few drops of acidified aqueous potassium manganate(VII). Record your observations. observations .................................................................................................................. [1] (ii) Carry out further tests to identify both anions in FB 7. Use a 1 cm depth of FB 7 in a test-tube for each test. Name the reagents you used and record the observations made in your tests. The anions present in FB 7 are ..................................... and ..................................... . [4] (b) FB 8 is a compound which contains one cation and one anion. Both ions are listed in the Qualitative analysis notes. (i) Transfer a small spatula measure of FB 8 into a hard-glass test-tube. Heat gently, then strongly, until no further change occurs. Allow the test-tube with the residue to cool for approximately 2 minutes. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Pour a 2 cm depth of dilute hydrochloric acid into a test-tube. Carefully, add a small spatula measure of FB 8 to the acid. Record your observations. Retain the mixture obtained for use in (b)(iii). ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Add an equal volume of distilled water to the mixture obtained from (b)(ii). Mix thoroughly. Carry out suitable tests to identify the cation in FB 8. Use a 1 cm depth of the solution in a test-tube for each test you carry out. Name the reagents you use. Record your observations. [2] (iv) From your observations in tests (b)(i), (b)(ii) and (b)(iii), deduce the formula of FB 8. FB 8 is ...................................... . [1] [Total: 11] Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+

Mark scheme: 3(a)(i) potassium manganate(VII) / KMnO4 is decolorised 1 or (purple solution) turns yellow / yellow-brown / orange-brown / amber / brown 3(a)(ii) reagents selected: 4 • (aqueous) silver nitrate / AgNO3 • (aqueous) barium nitrate / Ba(NO3)2 or (aqueous) barium chloride / BaCl2 observations: • (adding) AgNO3 gives a cream / off-white precipitate • precipitate is insoluble / partially soluble in (excess) ammonia • (adding) Ba(NO3)2 / BaCl2 gives a while precipitate • precipitate insoluble / not soluble in HCl / HNO3 conclusions: the anions in FB 7 are • Br − / bromide • SO42− / sulfate 3(b)(i) heating FB 8 2 • condensation (of liquid) • (solid) turns yellow / yellow-green or yellow(-green) solid formed • on cooling, (residue) becomes paler / turns white • attempts to test with limewater • gas / CO2 gives a white precipitate (with limewater) (There are 5 marking points. The last 2 may be awarded if the gas test and result is given in (b)(ii).) 3(b)(ii) fizzing / bubbling / effervescence 1 and either (reaction is) rapid / vigorous or solid dissolves / colourless solution formed 3(b)(iii) observations: 2 M1: add (aqueous) ammonia / NH3 giving white precipitate M2: ppt soluble / dissolves in excess (NH3) 3(b)(iv) FB 8 is ZnCO3 1

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