Cambridge A Level Chemistry 9701 — 2024 Oct/Nov Paper 3 · Variant 3

1 When a metal carbonate reacts with a suitable acid, carbon dioxide is produced. You will determine the relative formula mass, Mr, of a metal carbonate by reacting it with excess hydrochloric acid and measuring the mass of gas produced. FA 1 is the metal carbonate. FA 2 is 2.00 mol dm–3 hydrochloric acid, HCl. (a) Method ● Use the measuring cylinder to transfer 30.0 cm3 of FA 2 into a conical flask. ● Weigh the conical flask containing FA 2. Record the mass. ● Weigh the container with FA 1. Record the mass. ● Tip all the FA 1 slowly into the conical flask. When the reaction slows, swirl the flask gently. ● Weigh the container with any residual FA 1. Record the mass. ● Calculate the mass of FA 1 added. Record the mass. ● Leave the conical flask and its contents for 15 minutes. Swirl the flask occasionally during this time. During this period begin work on Question 2. ● After 15 minutes weigh the flask and its contents. Record the mass. I II III [3] (b) Calculations (i) Use your readings to calculate the mass of carbon dioxide produced. mass of CO2 = .............................. g [1] (ii) The ionic equation for the reaction of FA 1 with FA 2 is shown. CO32–(aq) + 2H+(aq) CO2(g) + H2O(l) Calculate the relative formula mass, Mr, of the metal carbonate FA 1. Show your working. Mr of FA 1 = .............................. [2] (c) (i) A student carries out the experiment as described in (a), except that the acid used is 15 °C colder than the acid you used. The student calculates the Mr correctly from the readings obtained. State whether the value of the Mr calculated by the student is higher or lower than the value you calculated in (b)(ii). Explain your answer. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Another student suggests that the experiment will be more accurate if the conical flask is tilted carefully to an almost horizontal position while the reaction is taking place. Explain why the student is correct. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (d) State the uncertainty in a single balance reading. uncertainty = ± .............................. g Give an expression that would enable you to calculate the percentage error in your weighing of FA 1. [1] [Total: 10]

Mark scheme: Question Answer Marks 1(a) I 5 headings and units for data 1 • (Mass of conical) flask + FA 2 / g (referred to as x in 1(b)(i)) • (Mass of) container + FA 1 / g. • (Mass of empty) container (or with residual FA 1) / g • (Mass) FA 1 (added) / g (referred to as y in 1(b)(i)) • (Total mass of conical) flask and contents at end / after 15 mins / after reaction / final / g (referred to as z in 1(b)(i)) II Precision and calculation 1 All four balance readings recording consistently either to 2 or to 3 d.p. and Subtraction for mass of FA 1 correct III Accuracy (Q) mark 1 Calculate supervisor to 2 d.p. = mass of FA 1 / mass loss Write this, in a ring, on each candidate’s script. Calculate the ratio for each candidate to 2 d.p. Upper and lower limits for the accuracy mark are the supervisor value plus or minus 15% and then rounded to 2 d.p. Award mark III if rounded candidate value is between rounded upper and lower limits (inclusive). 1(b)(i) Correct calculation of mass loss (= mass of CO2) 1 Mass loss = x + y – z (see 1(a) answer I) and answer given to 2–4 sig figs. 1(b)(ii) Correct use of (b)(i) to calculate Mr of FA 1. 2 M1 Correct expression to calculate no of moles of CO2 produced n(CO2) = (b)(i) / 44 M2 Correct use of data to calculate Mr mass of FA mass of FA 1  Mr = 1 / moles of CO2 = 44 / (b)(i) and answer given to 2–4 sig figs 1(c)(i) M1 (At lower temp) more CO2 dissolves in the solution 2 and less CO2 escapes / mass loss reduced OR The reaction is slower (at a lower temperature) and less CO2 produced / mass loss reduced M2 Lower amount / moles of CO2 (calculated) or lower amount / moles of (metal) carbonate (calculated) and higher Mr 1(c)(ii) (Tilting) allows air to replace / displace carbon dioxide in the flask 1 or (Tilting) tips out the (denser) carbon dioxide gas (from the flask) or (Tilting) allows the acid to react with solid stuck to the walls (owtte) of the conical flask. 1(d) Correct expression 1 Uncertainty U (for 2 d.p. balance) = 0.01 or 0.005 and % error in weighing FA 1 = 2  U/mass of FA 1 (used)  100

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2 The relative formula mass, Mr, of metal carbonate FA 1 can also be determined by titration of a solution of FA 1 with an acid such as hydrochloric acid. CO32–(aq) + 2H+(aq) CO2(g) + H2O(l) FA 3 is an aqueous solution of FA 1 containing 15.50 g dm–3 of the metal carbonate. FA 4 is a solution of hydrochloric acid containing 4.02 g dm–3 of HCl. FA 5 is bromophenol blue indicator. (a) Method ● Fill the burette with FA 4. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add a few drops of FA 5 into the same 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 all your burette readings and the volume of FA 4 added in each accurate titration. 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. 25.0 cm3 of FA 3 required .............................. cm3 of FA 4. [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 hydrochloric acid present in the volume of FA 4 calculated in (b). amount of HCl = .............................. mol [1] (iii) Calculate the concentration, in mol dm–3, of metal carbonate in FA 3. concentration of metal carbonate in FA 3 = .............................. mol dm–3 [1] (iv) Calculate the relative formula mass, Mr, of the metal carbonate in FA 1. Mr of metal carbonate = .............................. [1] (v) The metal carbonate in FA 1 is hydrated sodium carbonate, Na2CO3•xH2O. Calculate the value of x to the nearest whole number. Show your working. x = .............................. [1] (d) Describe a different method to determine the value of x in FA 1. This method should not involve the reaction of an acid. Explain how the method will ensure that the value of x is as accurate as possible. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] [Total: 15] 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 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 Correct headings and units 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 / FB 2 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 recorded to the nearest 0.05 cm3 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre Award accuracy Q marks as follows: V Award if  ⩽ 0.50 cm3 VI Award if  ⩽.0.30 cm3 VII Award if  ⩽ 0.20 cm3 If Supervisor’s mean titre > 50.00 cm3 then use the tolerances (0.30, 0.50, 0.80 cm3) If Supervisor’s mean titre < 10.00 cm3 then halve the tolerances (0.25, 0.15, 0.10 cm3) If Supervisor’s mean titre < 5.00 cm3 then use the tolerances (0.15, 0.10, 0.05 cm3) 2(b) Correctly calculates the mean titre to 2 decimal places. 1 • Candidate must take the average of two (or more) accurate 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. (e.g. 26.675 cm3 must be rounded to 26.68 cm3) 2(c)(i) All final numerical answers to parts 2(c)(ii), 2(c)(iii) and 2(c)(iv) are quoted to 3 or 4 significant figures. 1 2(c)(ii) Correct use of (b) 1 Amount of HCl used = 4.02 / 36.5  (b) / 1000 mol 2(c)(iii) Correct use of (c)(ii) 1 Concentration of Na2CO3 = (c)(ii) / 2  1000 / 25 mol dm–3 2(c)(iv) Correct use of (c)(iii) 1 Mr of Na2CO3 = 15.5(0) / (c)(iii) 2(c)(v) Correct use of (c)(iv) to calculate nearest whole number value of x 1 x = [(c)(iv) – 106] / 18 2(d) M1 Heat FA 1 to drive off water (of crystallisation) 2 or Heat FA 1 to convert it to anhydrous (salt) M2 (Heat to) constant mass (or description of procedure) OR Heat gently at first, with crucible lid on and to prevent solid frothing/spitting out of the crucible.

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3 (a) (i) FA 6 contains one cation and one anion both of which are listed in the Qualitative analysis notes. Heat a small spatula measure of FA 6 in a hard-glass test-tube, until no further change occurs. Record all your observations. Identify any gas produced. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (ii) Allow the residue to cool for 2 minutes. Then transfer a small quantity of the residue from (a)(i) into a test-tube containing a 2 cm depth of dilute sulfuric acid. Shake the test-tube. Record your observations. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Give the equation for the reaction in (a)(ii). Include state symbols. ..................................................................................................................................... [1] (b) FA 7 is a sample of the solution produced by the experiment described in (a)(ii). FA 8 is a solution of a salt containing one anion. (i) Use a 1 cm depth of FA 7 in a test-tube for each of the following tests. Record your observations in Table 3.1. Table 3.1 test observations Test 1 Add aqueous sodium hydroxide. Test 2 Add aqueous ammonia, then add dilute hydrochloric acid. Test 3 Add aqueous sodium carbonate. Test 4 Add several drops of FA 8, then add a few drops of aqueous starch. Test 5 Add a small spatula measure of iron powder. Leave the test-tube to stand. Test 6 Add a few drops of aqueous barium nitrate or aqueous barium chloride. [6] (ii) Carry out a different test from those you have already carried out in (b)(i) to confirm the identity of the anion in FA 8. Record the reagent(s) used and your observations and give the formula of the anion. The formula of the anion in FA 8 is .............................. . [2] (iii) List all the numbered tests you carried out in (b)(i) which involved redox reactions. ..................................................................................................................................... [1] (iv) Give the ionic equation for the reaction taking place in Test 5 in (b)(i). Include state symbols. ..................................................................................................................................... [1] [Total: 15]

Mark scheme: 3(a)(i) This part is ‘blob-marked’. Each available marking point is shown by a bullet. 3 • (FA 6 is) a (light) green / blue-green and solid / powder • (When heated) condensation / steam / water vapour produced • Solid fluidises / jumps about in test tube (owtte) • Black residue / solid formed • Attempts to test (gas) with limewater • (Gas / carbon dioxide) and limewater gives a white precipitate • Gas is carbon dioxide 3(a)(ii) (light) blue solution (formed) 1 3(a)(iii) CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) 1 3(b)(i) Observations 6 Test 1: blue ppt * insoluble in excess (NaOH) * Test 2: (pale) blue ppt * soluble / disappears in excess (NH3) * Deep / dark blue colour forms (with excess) * (With acid) solution becomes paler blue or blue ppt forms * Test 3: fizzing / effervescence * blue ppt (forms) * Test 4: (off-)white precipitate / solid (forms) * (with starch) goes dark blue / blue-black / black* Test 5: Brown solid / ppt forms * Fizzing or solution goes paler (blue) or solution goes colourless * (gas) pops with a lighted splint * Test 6: White precipitate (forms) * 3(b)(ii) FA 8: extra test 2 • Add (aqueous) silver nitrate / AgNO3 • (Pale) yellow precipitate formed • Precipitate is insoluble in (excess aqueous) ammonia • (FA8) contains I– OR • Add (acidified aqueous) potassium manganate (VII) / KMnO4 • Brown / yellow solution formed • Dark blue / blue-black / black colour with starch • (FA8) contains I– 3(b)(iii) 4 and 5 1 3(b)(iv) Cu2+(aq) + Fe(s) → Fe2+(aq) + Cu(s) 1 or Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g)

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