Cambridge A Level Chemistry 9701 — 2020 May/June Paper 3 · Variant 5

1 Ethanedioic acid forms salts with Group 1 metals. In this experiment you will identify the Group 1 metal ion, Z+, present in an ethanedioate salt, (COO)2Z2. You will titrate a solution of the salt with acidified aqueous potassium manganate(VII). The equation for the reaction between manganate(VII) ions and ethanedioate ions in acidic solution is shown. 2MnO4–(aq) + 16H+(aq) + 5(COO)22–(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g) FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is a solution containing 8.06 g dm–3 of an ethanedioate salt, (COO)2Z2. FA 3 is dilute sulfuric acid, H2SO4. (a) Method ●● Fill the burette with FA 1. ●● Pipette 25.0 cm3 of FA 2 into a conical flask. ●● Use the measuring cylinder to transfer 25 cm3 of FA 3 into the same conical flask. ●● Place the conical flask on the tripod and gauze and heat the conical flask until the temperature of the solution is approximately 70 °C. ●● Carefully remove the hot conical flask and place it on the white tile under the burette. ●● During titrations, add FA 1, slowly at first, until a permanent pale pink colour is formed. (The pink colour on initial addition may take several seconds to disappear.) If the reaction mixture turns brown, reheat it to approximately 70 °C. If the brown colour disappears, continue with the titration. If the brown colour remains, discard the contents of the flask and begin a new titration. ●● Perform a rough titration (the end‑point is a permanent pale pink colour) 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 of your burette readings and the volume of FA 1 added in each accurate titration. I II III IV V VI V I [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 required ............................... cm3 of FA 1. [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 manganate(VII) ions in the volume of FA 1 calculated in (b). moles of MnO4– = .............................. mol [1] (iii) Use the equation on page 2 to calculate the number of moles of ethanedioate ions in 25.0 cm3 of FA 2. moles of (COO)22– = .............................. mol [1] (iv) Calculate the relative formula mass, Mr, of the ethanedioate salt, (COO)2Z2. Mr of ethanedioate salt = .............................. [1] (v) Calculate the relative atomic mass, Ar, of the Group 1 metal, Z, in the ethanedioate salt. Show your working. Ar of Z = .............................. Hence identify Z. Z is .......................................................... . [2] [Total: 14]

Mark scheme: 1(a) I The following data is shown • two burette readings for the rough titration • titre for rough titration • initial and final burette readings for two (or more) accurate titrations 1 II Appropriate headings and units for accurate titration. and volume FA 1 added recorded for each accurate titre. Headings should match readings. • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FA 1 and used / added • unit: / cm3 or (cm3) or in cm3 or cm3 for each entry 1 III All accurate burette readings are to the nearest 0.05 cm3. 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award V, VI and VII if δ ⩽ 0.20 (cm3) Award V and VI if 0.20 < δ ⩽ 0.50 Award V, only, if 0.50 < δ ⩽ 0.80 3 1(b) Calculation of mean • Candidate must average two (or more) accurate titres where the total spread is ⩽ 0.20 cm3. • Working must be shown or ticks must be put next to the two (or more) accurate readings selected. • The mean should normally be quoted to 2 dp rounded to the nearest 0.01 cm3. (e.g.: 26.666 must be rounded to 26.67 cm3. 1 1(c)(i) All numerical answers in 1(c) are given to 3 or 4 sf 1 1(c)(ii) Correctly calculates 0.02 × (b) / 1000 1 1(c)(iii) Correctly uses answer (ii) × 5 / 2 (or equivalent) 1 Question Answer Marks 1(c)(iv) Correctly uses 8.06 / (40 × answer (iii)) 1 1(c)(v) Correct expression (answer (iv) – 88) ÷ 2 1 1(c)(vi) Correct answer K / potassium Allow ecf of group 1 metal ion: (Li < 15.0; 15.0 ⩽ Na <31.1; 31.1 ⩽ K ⩽ 62.3; 62.3 ⩽ Rb ⩽ 109.2; 109.2 ⩽ Cs) 1

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2 In this experiment you will determine the value of x in the formula for hydrated manganese(II) sulfate, MnSO4•xH2O, where x is an integer. You will do this by measuring the mass lost when a sample of hydrated manganese(II) sulfate is heated. MnSO4•xH2O(s) MnSO4(s) + xH2O(g) FA 4 is hydrated manganese(II) sulfate, MnSO4•xH2O. (a) Method ●● Weigh the crucible with a lid and record the mass. ●● Add all the FA 4 to the crucible. ●● Reweigh the crucible with the lid and FA 4. Record the mass. Describe the appearance of FA 4. appearance of FA 4 ............................................................................................................. ●● Place the crucible in the pipe-clay triangle on top of the tripod. ●● Heat the crucible gently with the lid on for approximately 1 minute. ●● Remove the lid and then heat more strongly for a further 4 minutes. ●● Replace the lid and allow the crucible to cool. ●● While the crucible is cooling you may wish to begin work on Question 3. ●● Once the crucible has cooled, reweigh the crucible with the lid and contents. Record the mass. ●● Calculate and record the mass of FA 4 added to the crucible, the mass of the residue and the mass of water lost. ●● Describe the appearance of the residue. appearance of the residue .................................................................................................. I II III IV V VI [6] (b) Calculations (i) Calculate the number of moles of manganese(II) sulfate present in the residue. You may assume all the water of crystallisation has been removed. moles of MnSO4 = .............................. mol [1] (ii) Calculate the number of moles of water lost. moles of water lost = .............................. mol [1] (iii) Calculate the value of x in MnSO4•xH2O. x = .............................. [1] (c) It is possible that FA 4 did not lose all of the water of crystallisation in your experiment. (i) Explain how you could modify the experiment to ensure all water has been removed. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Explain why your calculated value of x might not change if a small amount of water of crystallisation remained in the residue. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [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 Both colours and states correct: (from) (pale) pink crystals / solid to off-white / beige / buff / (pale) brown powder / solid 1 II Table of data Must show all of the following: • mass of crucible (+ lid) • mass of crucible (+ lid) + FA 4 • mass of crucible (+ lid) + residue • mass of FA 4 • mass of residue • mass of water lost 1 III Recording of data • Unit / g, (g) or in grams • all three balance readings recorded to same number of dp 1 IV Calculations Calculates correctly the mass of FA 4, mass of residue and mass of water lost. 1 2(b)(i) Correctly calculates mol MnSO4 = mass residue ÷ 151(.0) and answer to 2–4 sf 1 2(b)(ii) Correctly calculates mol H2O = mass water lost ÷ 18 and answer to 2–4 sf 1 2(b)(iii) Correctly uses ans (ii) ÷ ans (i) and x is an integer 1 2(c)(i) Heat to constant mass 1 2(c)(ii) The changes in masses (of residue and water) will be too small to change the integer value. 1

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3 Half fill the 250 cm3 beaker with water and place it on a tripod and gauze. Heat until the water begins to boil then switch off your Bunsen burner. This is the hot water bath for part (b). (a) FA 5 is a solution of a salt which contains one cation and at least one anion, all of which are listed in the Qualitative Analysis Notes. Sulfur is not present in FA 5. (i) To a 1 cm depth of FA 5 in a test‑tube add aqueous sodium hydroxide. observation .......................................................................................................................... ....................................................................................................................................... [1] (ii) You are to carry out tests to allow you to determine the anion present in FA 5. Identify reagents for these tests, carry out these tests and record these tests and observations in a table. [3] (iii) Use your results to identify the ions present in FA 5. formulae of ions present in FA 5 ................................................................................... [1] (iv) Write an ionic equation for the expected reaction between FA 5 and aqueous ammonia. Include state symbols. ....................................................................................................................................... [1] (v) Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of FA 5 in a test‑tube, add a 1 cm depth of hydrogen peroxide, then add aqueous sodium hydroxide. [2] (vi) Suggest what type of reaction occurred when hydrogen peroxide was added to FA 5. ....................................................................................................................................... [1] (b) FA 6, FA 7 and FA 8 are butan‑1‑ol, butan‑2‑ol and methylpropan‑2‑ol, but not necessarily in that order. (i) Carry out the following tests and record your observations. observations test FA 6 FA 7 FA 8 Test 1 To a 1 cm depth of dilute sulfuric acid in a test‑tube, add 2 or 3 drops of FA 1, KMnO4, then add a few drops of the alcohol. Shake the tube and place it in the hot water bath. Shake the tube occasionally until there is no further change. Test 2 To a 1 cm depth of aqueous iodine in a test‑tube, add a few drops of the alcohol, then add drops of aqueous sodium hydroxide until the iodine colour just disappears or remains unchanged. Place the test‑tube in the hot water bath. [3] (ii) Use your observations from (b)(i) to identify the alcohols. alcohol FA…. butan-1-ol FA…........... butan-2-ol FA…........... methylpropan-2-ol FA…........... [2] (iii) Write an equation for the oxidation of one of these alcohols with acidified KMnO4. Use [O] to represent the oxidising agent. ....................................................................................................................................... [1] [Total: 15]

Mark scheme: 3(a)(i) Off-white / pale brown / buff / beige ppt and insoluble in excess / ppt darkens / ppt turns brown / darker brown 1 3(a)(ii) Selects (aqueous) silver nitrate and (aqueous) sodium hydroxide with aluminium and either mention of heating or testing gas with red litmus Selection of any other reagent, Ba2+, negates this mark. 1 Clear test / observation sections suitable for two tests 1 White ppt with Ag+ and no ammonia with Devarda’s test 1 3(a)(iii) FA 5 = Mn2+ and Cl - 1 3(a)(iv) Mn2+(aq) + 2OH‒(aq) → Mn(OH)2(s) 1 3(a)(v) 3 observations = 1 mark test residue + H2O2 no visible reaction + NaOH dark brown ppt / solid AND effervescence / bubbling / fizzing 1 (gas / O2 / fizz) relights glowing splint / makes splint glow brighter 1 3(a)(vi) redox / disproportionation / catalysis 1 Question Answer Marks 3(b)(i) Award one mark for every two correct observations test FA 6 FA 7 FA 8 + H+ / MnO4‒ no reaction / no change / remains purple purple decolourised allow turns (pale) yellow-brown / brown purple decolourised + I2 / NaOH no reaction / no change / no ppt no reaction / no change / no ppt pale yellow ppt 3 3(b)(ii) alcohol FA… butan-1-ol FA 7 butan-2-ol FA 8 methylpropan-2-ol FA 6 3 correct award 2 marks, 1 correct award 1 mark 2 3(b)(iii) CH3(CH2)2CH2OH + [O] → CH3(CH2)2CHO + H2O or CH3(CH2)2CH2OH + 2[O] → CH3(CH2)2COOH + H2O or CH3CH2CHOHCH3 + [O] → CH3CH2COCH3 + H2O 1

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