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

1 An iron compound can contain Fe2+ ions, Fe3+ ions or both ions. In this experiment, you will determine the percentage by mass of iron in FA 1, an unknown compound. You will first prepare a solution of the compound and then carry out a titration using acidified potassium manganate(VII), KMnO4. 5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) The end-point of the titration is when all the Fe2+ ions have been oxidised and so unreacted potassium manganate(VII) causes the colour of the solution to become a permanent pale pink. FA 1 is a sample of the unknown compound. FA 2 is 0.0100 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is dilute sulfuric acid, H2SO4. (a) Method Weighing the compound ● Weigh the sample of FA 1 and its container. ● Tip the FA 1 into the 250 cm3 beaker. ● Reweigh the container including any residual FA 1. ● Record both your readings clearly in the space below. ● Calculate the mass of FA 1 transferred into the beaker. Record the mass. Preparing the solution ● Add approximately 200 cm3 of distilled water to the beaker and stir until the FA 1 has dissolved. ● Pour the contents carefully into the 250 cm3 volumetric flask. ● Rinse the contents of the beaker with a little distilled water and add these washings to the flask. ● Fill the flask to the line with distilled water and shake thoroughly. ● Label this solution FA 4. Titration ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 4 into a conical flask. ● Use the 25 cm3 measuring cylinder to add 15 cm3 of FA 3 to the conical flask. ● Add FA 2 from the burette until the solution in the flask turns a permanent pink. ● Carry out a rough titration and record your burette readings in the space below. rough titre = .............................. cm3 ● Carry out as many accurate titrations as you think are necessary to obtain consistent results. ● Make sure that your recorded results show the precision of your practical work. ● Record in a suitable format, in the space below, all your burette readings and the volume of FA 2 added in each accurate titration. I II III IV V VI VII VIII [8] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 4 required .............................. cm3 of FA 2. [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 manganate(VII) ions in the volume recorded in (b). amount of MnO4– = .............................. mol [1] (iii) Calculate the amount, in mol, of iron(II) ions in the weighed sample of FA 1. amount of Fe2+ = .............................. mol [1] (iv) Calculate the percentage by mass of iron in FA 1. Show your working. percentage by mass of iron = .............................. % [2] (d) A student carries out the same experiment as in (a). The student receives a sample of FA 1 in a container with a lid. The student records the initial mass of the container with its lid and the sample of FA 1. Then the student records the mass of the container with the residue but forgets to replace the lid. How would this error alter the student’s answer to (c)(iv)? Explain your answer. .................................................................................................................................................... .............................................................................................................................................. [1] (e) State two assumptions that have been made in calculating the percentage by mass of iron in FA 1 in (c)(iv). .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 17] Question 2 starts on the next page.

Mark scheme: Question Answer Marks 1(a) I table or list to include: 8 • initial and final balance readings to same precision and to 2 or 3 dp • correctly calculated mass of FA 1 II the following data must be shown: • two burette readings and titre for rough titration • initial final burette readings for two (or more) accurate titrations III 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 / FA 2 and used / added unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded IV all accurate burette readings are to nearest 0.05 cm3 V the final accurate titre recorded is within 0.10 cm3 of any other accurate titre VI, VII, VIII award VI if  ⩽ 0.80 (cm3) award VII if  ⩽ 0.50 (cm3) award VIII if  ⩽ 0.30 (cm3) where d is the difference between the supervisor’s mean titre and the candidate’s scaled mean titre supervisor's mass used = candidate’s scaled mean titre  (calculated to 2 decimal places (dp)) candidate's mass used 1(b) candidate calculates mean correctly to 2 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 titres selected • the mean should be quoted to 2 dp and be rounded to the nearest 0.01 cm3 1(c)(i) Significant figures 1 all quoted answers in (c)(ii) – (c)(iv) are expressed to 3 or 4 sf. 1(c)(ii) correctly calculates 1 0.01× vol ( b ) amount of MnO4− = (mol) 1000 1(c)(iii) correctly uses 1 5  (c)(ii) AND 10  ANS 1(c)(iv) M1: correct expression 2 display of (c)(iii)  55.8 M2: correctly uses  ( c )( iii ) × 55.8  percentage by mass of iron =    100 (%)  mass in ( a )  1(d) the (apparent) mass of the sample would be higher 1 AND the percentage would decrease 1(e) M1: all the iron is present as Fe2+ ions / no Fe3+ present 2 M2: only Fe2+ / iron ions react with KMnO4 OR the unknown anion / (any other) cation does not react with KMnO4 OR there are no impurities that react with KMnO4

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2 In this experiment you will determine the percentage by mass of magnesium in a hydrated salt, MgX•7H2O, where X represents the anion. You will measure the loss of mass when a sample of the hydrated salt is heated to form the anhydrous salt. FA 5 is a pure sample of MgX•7H2O (a) Method ● Weigh the crucible with its lid. Record the mass. ● Add all of FA 5 to the crucible. ● Reweigh the crucible with its lid and FA 5. Record the mass. ● Support the crucible in the pipe-clay triangle on top of the tripod. ● Heat the crucible with the lid on gently for about 1 minute. ● Remove the lid and then heat strongly for a further 4 minutes. ● Replace the lid and allow the crucible to cool for at least 5 minutes. While the crucible is cooling you may wish to begin work on Question 3. ● When the crucible is cool enough to handle, reweigh the crucible with its lid and its contents. Record the mass. ● Calculate and record the mass of FA 5 used. ● Calculate and record the mass of water lost. I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of water lost. amount of H2O lost = .............................. mol [1] (ii) Calculate the percentage by mass of magnesium in FA 5. Show your working. percentage by mass of Mg = .............................. % [2] (c) Suggest two assumptions that must be made for this experiment to give an accurate value of the percentage of Mg. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 10] 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 table of data 5 must show all of the following: • (mass of) crucible + lid • (mass of) crucible + lid + FA 5 • (mass of) crucible + lid + residue / contents / MgX • (mass of) FA 5 • (mass of) water lost / mass lost II recording of data • unit / g, (g) or in grams is covering each entry • all three balance readings are recorded to the same number of decimal points (2 or 3 dp). III correctly calculates • mass of FA 5 • mass of water lost IV, V award IV if  ⩽ 0.20 award V if  ⩽ 0.10 mass of FA 5 where d is the difference between the supervisor’s and candidate’s mass ratio = and calculated correct mass of water lost to 2 dp the theoretical value of 1.96 is used if there is no supervisor’s value available 2(b)(i) correctly calculates 1 mass of water lost amount of H2O lost = (mol) 18 AND answer to 2 – 4 sf 2(b)(ii) correctly uses 2 M1: amount of Mg = (b)(i) / 7 (mol) M2: percentage of Mg = [{(b)(i)  24.3} / (7  mass of FA 5)]  100 (%) AND answer to 2 – 4 sf 2(c) M1: all the water is lost. 2 OR heating was for long enough to reach constant mass. (owtte) M2: MgX / residue is thermally stable / does not decompose. OR the mass loss is due to the water only.

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3 (a) FA 6 is an acidified aqueous solution of a salt which contains two cations and one anion, all of which are listed in the Qualitative analysis notes. (i) Select a reagent or reagents for use in tests to identify the two cations. Record your observations. [2] (ii) The anion in FA 6 is either the sulfate ion, SO42–, or the sulfite ion, SO32–. Select a reagent or reagents for use in tests to identify the anion. Record your observations. [2] (iii) Identify the ions present in FA 6 and give their formulae. The cations present in FA 6 are .............................. and .............................. . The anion present in FA 6 is .............................. . [2] (b) Carry out the tests and record your observations. For each test use a 1 cm depth of FA 6 in a test-tube. test observations Test 1 Add a 1 cm depth of aqueous sodium sulfite, then add a 1 cm depth of dilute sulfuric acid. Rinse out this test-tube thoroughly. Test 2 Add a 1 cm depth of aqueous potassium iodide, then add a few drops of starch solution. [2] (c) FA 7, FA 8 and FA 9 are aqueous solutions of sodium salts. Each solution contains one of the following ions: CO32–, Cl –, Br –. You will identify which solution contains which ion. (i) Carry out the tests and record your observations. For each test use a 1 cm depth of FA 7, FA 8 or FA 9 in a test-tube. observations test FA 7 FA 8 FA 9 Add a few drops of aqueous silver nitrate, then add aqueous ammonia. [3] (ii) If your results are insufficient to identify which anion is present in each solution, carry out a further test. Record your test and observations. [1] (iii) Identify which solution contains which ion. The CO32– ion is present in .............................. . The Cl – ion is present in .............................. . The Br – ion is present in .............................. . [1] [Total: 13]

Mark scheme: 3(a)(i) M1: selects NaOH OR NH3 2 AND red-brown / brown / rust / orange-brown ppt on adding NaOH or NH3 M2: warms solution with NaOH AND ammonia / gas (given off on warming) that turns (damp red) litmus blue. 3(a)(ii) M1: selects (acidified) potassium manganate(VII) / KMnO4 (with or without barium salt) 2 OR selects BaCl 2 or Ba(NO3)2 AND HCl or HNO3 (in either order) (or full names) M2: (KMnO4) solution stays purple / no change / no reaction OR white ppt AND insoluble in (excess) acid 3(a)(iii) identifies ions from (partially) successful tests: 2 cations NH4+ Fe3+ anion SO42– Any 2 ions correct = 1 mark All ions correct = 2 marks 3(b) Test 1 2 + Na2SO3(aq) • forms red solution / (pale) yellow solution turns red + H2SO4 • (solution turns) colourless / pale yellow Test 2 + KI(aq) • (solution turns) darker yellow / yellow-brown / brown / colourless (solution) turns yellow + starch • turns blue-black / black 2 • = 1 mark 3(c)(i) FA 7: 3 • cream / off-white ppt with AgNO3(aq) • insoluble / partially soluble in NH3(aq) FA 8: • white / off-white / cream / brown ppt with AgNO3(aq) • soluble in NH3(aq) / forms colourless solution FA 9: • white ppt with AgNO3(aq) • soluble in NH3(aq) / forms colourless solution 3(c)(ii) one of: 1 selects named acid AND observes fizzing with FA 8 or limewater gives white ppt with gas / CO2 OR tests solution(s) with litmus paper or universal indicator AND only FA 8 turns (red) litmus blue or (yellow / green) UI turns blue 3(c)(iii) all identities are correct 1 the CO32– ion is present in FA 8 the Cl – ion is present in FA 9 the Br – ion is present in FA 7

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