Cambridge A Level Chemistry 9701 — 2021 May/June Paper 3 · Variant 3

1 Bleach is made by reacting chlorine with a cold solution of sodium hydroxide. This reaction produces sodium chlorate(I), NaCl O. Cl 2(aq) + 2NaOH(aq) → NaCl O(aq) + NaCl (aq) + H2O(l) In this experiment you will determine the concentration of sodium chlorate(I) in a sample of bleach, FA 1. To do this, you will react an acidified dilute solution of the bleach with iodide ions, I–. This reaction produces iodine, I2. Cl O–(aq) + 2I–(aq) + 2H+(aq) → I2(aq) + Cl –(aq) + H2O(l) The amount of iodine produced will then be determined by titration with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) FA 1 is a solution of bleach. FA 3 is dilute sulfuric acid, H2SO4. FA 4 is 0.500 mol dm–3 potassium iodide, KI. FA 5 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Method Dilution ● Pipette 25.0 cm3 of FA 1 into the 250 cm3 volumetric flask. ● Add distilled water to make 250 cm3 of solution and shake the flask thoroughly. ● Label this flask FA 2. Titration ● Fill a burette with FA 5. ● Rinse the pipette thoroughly with distilled water and then with a little FA 2. ● Pipette 25.0 cm3 of FA 2 into a conical flask. ● Use the measuring cylinder to add 20 cm3 of FA 3 to the conical flask. ● Use the measuring cylinder to add 15 cm3 of FA 4 to the conical flask. The solution will turn brown as iodine is produced. ● Add FA 5 from the burette until the solution has turned yellow. ● Add 10 drops of starch indicator to the conical flask. The solution will turn blue‑black. ● Continue to add more FA 5 from the burette until the blue-black colour just disappears. This is the end‑point of the titration. ● Carry out a rough titration and record your burette readings in the space provided. The rough titre = .............................. cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure that your 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 FA 5 added in each accurate titration. Keep FA 4 and FA 5 for use in Question 3. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a value for the volume of FA 5 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 required .............................. cm3 of FA 5. [1] (ii) Use your answer to (b) and the relevant equation on page 2 to calculate the number of moles of iodine that formed when 25.0 cm3 of FA 2 reacted with FA 4. moles of I2 = .............................. mol [1] (iii) Calculate the concentration of sodium chlorate(I) in FA 1. Show your working. concentration of NaCl O = .............................. mol dm–3 [2] (d) (i) In this method an excess of potassium iodide must be added to 25.0 cm3 of FA 2. Use your answer to (c)(ii) to show by calculation that the potassium iodide is in excess. [1] (ii) A student carries out the same method but the concentration of the potassium iodide solution is not stated. What change to the practical procedure could the student make to check that the potassium iodide is in excess? Explain your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] [Total: 15]

Mark scheme: 1(a) l The following data must be shown: • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings 1 II Headings and units correct for accurate titration table and headings match readings. • initial/start and (burette) reading / volume + unit • final/end and (burette) reading / volume + unit • titre or volume/FA 5 and used / added + unit Units: (cm3) or / cm3 or in cm3 or cm3 by every entry 1 III All accurate burette readings to 0.05 cm3 1 Question Answer Marks 1(a) IV The final accurate titre recorded is 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. Examiner selects the best mean titre. Apply hierarchy: 2 identical, titres within 0.05 cm3, titres within 0.10 cm3 etc. Examiner subtracts (corrected) candidate’s titre from Supervisor’s titre. Write and ring supervisor’s value next to the accurate titration table of each candidate, also candidate mean value (calculated by examiner) and difference between supervisor’s and candidate’s value, δ. Award accuracy marks as follows: V Award if δ ⩽ 0.60 cm3 VI Award if δ ⩽ 0.40 cm3 VII Award if δ ⩽ 0.20 cm3 (3) 7 1(b) Correctly calculates mean accurate titre • 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. 1 1(c)(i) Answers for (c)(ii) and (iii) to 3 or 4 sf 1 1(c)(ii) Correctly calculates amount of I2 = [(b) × 0.1] / [2 × 1000] (= (b) × 5 × 10−5 mol) 1 Question Answer Marks 1(c)(iii) Correctly uses: If candidate’s answer = 400 × (c)(ii) and working shown award both marks If candidate’s answer = 400 × (c)(ii) no working shown award 1 mark If candidate’s answer = 200 × (c)(ii) award 1 mark (I2 / I− confusion) If candidate’s answer = 100 × (c)(ii) award 1 mark (250 / 25 instead of 1000 / 25) If candidate’s answer = 40 × (c)(ii) award 1 mark (dilution factor missing) 2 1(d)(i) moles of KI = 15 / 1000 × 0.5 = 7.5 × 10–3 mol moles I− = (c)(ii) × 2 2 x (c)(ii) < 7.5 × 10–3 OR a comparison in words ((c)(ii) < 3.75 × 10–3) 1 1(d)(ii) M1: Repeat the experiment using a greater volume of KI M2: If titre value is the same then KI was in excess OR M1: Add more KI at the endpoint M2: If mixture / it goes black / blue-black / dark blue then KI was not in excess OR M1: Add AgNO3 before titrating M2: Yellow ppt observed shows presence of I− / no yellow ppt shows I− not in excess OR M1: Add more/extra NaClO when the titration mixture is yellow M2: if the mixture goes darker yellow / brown etc., (this shows more iodine has been produced, as) KI is in excess. (ORA) 2

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2 In this experiment you will determine the value of x in the formula for hydrated sodium thiosulfate, Na2S2O3•xH2O. In this formula x is an integer. FA 6 is hydrated sodium thiosulfate, Na2S2O3•xH2O. (a) Method ● Support the cup in the 250 cm3 beaker. ● Rinse the measuring cylinder thoroughly with distilled water. ● Use the measuring cylinder to transfer 20.0 cm3 of distilled water into the cup. ● Measure and record the initial temperature of the water in the cup. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. ● Weigh the stoppered container of FA 6 and record the mass. ● Carefully add all the sample of FA 6 to the water in the cup. ● Stir the mixture and record the minimum temperature. ● Reweigh the stoppered container and any residual FA 6. Record the mass. ● Calculate and record the mass of FA 6 added to the water and the change in temperature. I II III IV [4] (b) Calculations (i) Calculate the heat energy change in the reaction. (Assume that 4.2 J are required to change the temperature of 1.0 cm3 of solution by 1.0 °C.) heat energy change = .............................. J [1] (ii) Use your answer to (b)(i) to calculate the relative formula mass, Mr, of hydrated sodium thiosulfate. You should assume that the enthalpy change of hydration, ∆Hhyd, for Na2S2O3•xH2O is +47.4 kJ mol–1 under the conditions of your experiment. Show your working. Mr of Na2S2O3•xH2O = .............................. [2] (iii) Use your answer to (b)(ii) to calculate the value of x in the formula Na2S2O3•xH2O. x = .............................. [1] (c) A student repeats the experiment but adds 25.0 cm3 of distilled water into the cup instead of 20.0 cm3. The student carries out the calculations based on adding 20.0 cm3. What effect would this have on the student’s answer to (b)(ii)? Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 10] 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 Table / list to include initial and final balance reading, initial and final thermometer reading, mass added and temperature change. and all entries with correct units: / °C, (g) or in °C or in degrees Celcius, in g or in gram(me)s and recorded in the space provided 1 II All balance readings recorded to the same precision (minimum 1 dp) and all thermometer readings recorded to .0 or .5 °C. 1 Accuracy marks Examiner checks and corrects mass of FA 6 used and temperature fall (round thermometer readings to the nearest .0 or .5 ºC). Examiner calculates ΔT / mass to 2 dp for supervisor and candidate Write and ring supervisor’s ratio next to the results table of each candidate, also candidate ratio and difference between supervisor’s and candidate’s value, δ. Award accuracy marks as follows: III Award if δ ⩽ 0.40 °C g−1 IV Award if δ ⩽ 0.20°Cg−1 (2) 4 2(b)(i) Correctly calculates heat energy change = 20 × 4.2 × temperature change and answer given to 2–4 sf 1 2(b)(ii) M1: Correct display of mass of FA 6 / moles of thiosulfate or (b)(i) / 47400 M2: Correct use of (correct mass FA 6 × 47400) / ans (b)(i) and answer given to 2–4 sf 2 Question Answer Marks 2(b)(iii) Correct display x = [(b)(ii) – 158.2] / 18 and answer given to the nearest integer 1 2(c) M1: temperature fall / change would be less OR heat energy change would be less M2: therefore moles/ amount of thiosulfate would be fewer / less and so Mr would be greater 2

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3 (a) FA 7 is a salt containing one cation and one anion, both of which are listed in the Qualitative Analysis Notes. (i) Transfer a small spatula measure of FA 7 to a hard‑glass test‑tube and heat, gently at first and then more strongly. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Place the remaining FA 7 in the 100 cm3 beaker and dissolve in approximately 50 cm3 of distilled water. Use a sample of this solution to test for the cation present in FA 7. Record your tests and observations in the space below. Keep the solution for use in (a)(iii). [3] (iii) To a 1 cm depth of solution FA 7 in a test‑tube, add a few drops of aqueous silver nitrate. Record your observations. ....................................................................................................................................... [1] (iv) Suggest the ions present in FA 7. cation ............................................................ anion ........................................................... [1] (b) FA 8 is acidified aqueous iron(III) chloride, FeCl 3. Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of FA 8 in a test-tube, add a 1 cm depth of FA 4, then add starch indicator. Test 2 To a 1 cm depth of FA 8 in a test‑tube, add a 1 cm depth of FA 5. Allow to stand until there is no further change, then add a 1 cm depth of FA 4, then add starch indicator. [3] (c) Half fill the 250 cm3 beaker with water. Place the beaker on a gauze supported on the tripod and heat the water to approximately 70 °C. Switch off the Bunsen burner. This will be used as a water‑bath. FA 9, FA 10 and FA 11 are each one of the following. ● 1.0 mol dm–3 hydrochloric acid, HCl ● 1.0 mol dm–3 methanoic acid, HCOOH ● 1.0 mol dm–3 sodium sulfite, Na2SO3 (i) Carry out the following tests and record your observations. For each test use a 1 cm depth in a test‑tube. observations test FA 9 FA 10 FA 11 Test 1 Add a 1 cm strip of magnesium. Test 2 Add a few drops of aqueous acidified potassium manganate(VII). Place the test‑tube in the hot water‑bath. [3] (ii) Identify each of the solutions. FA 9 is ................................................... . FA 10 is ................................................. . FA 11 is ................................................. . [2] [Total: 15]

Mark scheme: 3(a)(i) 2 * = 1 mark (round down) white solid / powder / crystals (at start) * forms liquid/ melts / dissolves (in water of crystallization) * condensation / water droplets (on side of test-tube) / water vapour * white residue / solid / powder forms * NOT ppt (blue) litmus turns red * 3(a)(ii) M1: Selects NaOH and NH3 ONLY M2 & 3: + NaOH: white ppt * which is insoluble in excess * + NH3: white ppt * which is insoluble in excess * 2 * = 1 mark (round down) 3 3(a)(iii) + AgNO3: white ppt 1 Question Answer Marks 3(a)(iv) cation: magnesium / Mg2+ and anion: chloride / Cl – (from white ppt) 1 3(b) Test 1. M1: + FA 4 yellow (solution) turns darker yellow / yellow-brown / orange-brown / red-brown / brown M2: + starch – (turns) black / blue-black / dark blue Test 2. M3: + FA 5 solution turns purple and colour fades on standing 3 3(c)(i) Test 1 FA 9 effervescence / bubbling / fizzing * FA 10 no reaction / no effervescence / bubbling / fizzing * FA 11 faster effervescence / bubbling / fizzing * gas / H2 pops with a lighted splint with FA 9 and / or FA 11 * Test 2 FA 9 KMnO4 decolourised OR purple to / turns colourless * FA 10 KMnO4 decolourised OR purple to / turns colourless * FA11 no reaction OR solution remains purple * 2* = 1 mark (round down) 3 Question Answer Marks 3(c)(ii) FA 9 = HCOOH / methanoic acid; FA 10 = Na2SO3 / sodium sulfite / sodium sulfate(IV) FA 11 = HCl / hydrochloric acid 1 mark for one correct 2 marks for all three correct 2

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