Cambridge A Level Chemistry 9701 — 2020 Oct/Nov Paper 3 · Variant 1

1 In this experiment you will determine the value of x in the formula of hydrated sodium thiosulfate, Na2S2O3•xH2O, where x is an integer. You will first prepare a solution of the salt and then use this solution in a titration with aqueous iodine. The thiosulfate ions react with iodine as shown. 2S2O32–(aq) + I2(aq) → S4O62–(aq) + 2I–(aq) FA 1 is hydrated sodium thiosulfate, Na2S2O3•xH2O. FA 3 is 0.0500 mol dm–3 iodine, I2. starch indicator (a) Method Preparation of salt solution ● Weigh the container containing FA 1. ● Tip the contents of the container into the 250 cm3 beaker. ● Weigh the container with any residue. ● Record all your readings in the space below. ● Add approximately 200 cm3 of distilled water to the salt in the beaker and stir until the salt has dissolved. ● Pour the contents carefully into the 250 cm3 volumetric flask. ● Rinse the beaker with a little distilled water and add these washings to the flask. ● Fill the flask to the mark with distilled water and shake to ensure thorough mixing. ● Label this solution FA 2. Titration ● Fill a burette with FA 2. ● Pipette 25.0 cm3 of FA 3 into the conical flask. ● Add FA 2 from the burette until the solution in the flask turns yellow. ● Add 10 drops of starch indicator to the conical flask. The solution will turn blue-black. ● Continue to add more FA 2 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 below. The rough titre is .............................. cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure your recorded results show the precision of your practical work. ● Record, in a suitable form in the space below, all of 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, obtain a value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required ............................... cm3 of FA 2. [1] (ii) Calculate the number of moles of iodine in 25.0 cm3 of FA 3. moles of I2 = .............................. mol [1] (iii) Calculate the number of moles of thiosulfate ions in the volume recorded in (b). moles of S2O32– = .............................. mol Hence calculate the number of moles of hydrated sodium thiosulfate in the mass weighed in (a). moles of Na2S2O3•xH2O = .............................. mol [1] (iv) Calculate the value for x in the formula of hydrated sodium thiosulfate, Na2S2O3•xH2O. Show your working. x = .............................. [3] Calculate the maximum percentage error in the mass of FA 1 used in (a). Show your working. maximum percentage error = ± .............................. % [1] (ii) Assume that the uncertainty in the mass of FA 1 is the only source of error in your experiment. Calculate the minimum value for the relative formula mass of FA 1. Show your working. minimum value for the relative formula mass of FA 1 = .............................. [1] (e) A student prepares FA 2 using anhydrous sodium thiosulfate salt and the same mass of salt that you used in (a). State how the student’s titre would compare with the average titre value you obtained in (b). Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (f) In many titrations it is usual to fill the burette with the solution of known concentration. Suggest why this was not done in (a). .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 19]

Mark scheme: 1(a) I Four (or more) experiments completed AND Table on page 4 with correct headings showing • volume of FA 1 • volume of water • time • rate Allow vol. Ignore V or FA 1 alone. Ignore extra columns of data. Ignore data for this mark. II Correct units for all data (in heading or for each entry) • volume: in cm3 or / cm3 or (cm3) or cm3 • time: / s or (s) or s by each time Allow in seconds • rate: / s–1 or (s–1) or s–1 Ignore factor of 1000 1 III All times recorded to nearest second (minimum of 3 times) AND volumes of FA 1 and water recorded to the nearest 0.05 cm3 1 IV Three additional experiments with volume FA 1 not less than 6.00 cm3, not more than 20.00 cm3 and no volume less than 2.00 cm3 close to another volume. Reject if further additional experiments carried out. Reject if all 3 additional experiments are between 20 and 10 cm3. 1 V Volumes of water chosen so that FA1 + water = 20.00 cm3 for additional experiments carried out. Reject if FA 1 = 0 Reject if no times recorded. 1 Question Answer Mark 1(a) VI Correctly calculates rate for all experiments and answer shown to 2–4 sf. Use of significant figures or decimal places does not have to be consistent. Allow for a minimum of 3 experiments attempted. 1 Round times to the nearest second before awarding Q marks. Convert times recorded as decimals to seconds, e.g.1.42 (1:42) = 102 s VII Award if all candidate’s times increase with decrease in volume of FA 1. Reject if fewer than 4 experiments carried out. 1 Calculate candidate’s ratio = = time for 10 time 20 FA 1 FA 1 to 2 dp and record the value under the results table. VIII Award if ratio is between 3.20 and 4.80 1 IX Award if ratio is between 3.50 and 4.50 1 X Award if ratio is between 3.80 and 4.20 1 1(b) I Linear scales that cover more than half the space in both directions including (0,0) AND axes correctly orientated and clearly labelled If scale is non-linear then II is not available. 1 II Points plotted correctly. Points must be within half a small square of the correct position. If the point should be on a line it must be on the line and if it should not be on the line it must not be so. ‘Blobs’ should be less than half a small square across and be correctly centred. Reject if the scale is non-linear. 1 Question Answer Mark 1(b) III Line of best fit drawn which ignores anomalous results identified by the candidate. The line may be a smooth curve or straight AND use a minimum of 3 points. Ignore points which are circled or labelled as anomalous. Reject if a point has been shown at the origin and the line of best fit does not pass within 5 small squares of (0,0). 1 1(c) Correct lines drawn within 1 small square Allow if horizontal line drawn and some mark shown at 5. 1 Rate must be read to within correct half a small square, compared with examiner-read value. AND Correctly calculates 1000 / rate AND answer correct to 2–4 sf or a whole number of seconds (unless penalised in 1(a)VI). Reject if the portion of the scale used for the reading is non-linear. 1 1(d)(i) Correctly calculates Expt 1: 20 cm3 in total volume 60 cm3 ⇒ 1.67 × 10‒2 mol dm‒3 Expt 2: 10 cm3 in total volume 60 cm3 ⇒ 0.83 × 10‒2 mol dm‒3 1 = 20 1.67 10 0.83 so directly proportional Allow directly proportional as total volume unchanged but volume of FA 1 / KI doubled / halved. 1 If neither mark is awarded, then allow a total of one mark in (d)(i) for a correct calculation of moles of KI in Experiments 1 and 2 (1 × 10–3 and 5 × 10–4 mol respectively). Question Answer Mark 1(d)(ii) Graph: comment must refer to the shape of the line drawn. Curve: as concentration / volume (of iodide ions) increases rate increases more / not directly proportional as line is a curve / not a straight line Straight line: rate is proportional to concentration (of iodide ions) / proportional as line has a positive gradient Reject directly proportional unless the straight line passes within 5 small squares of (0,0). OR Table: compares ratio of concentrations / volumes of FA 1 with ratio of rates 1 Reason: (plotted points give) line of increasing gradient (This mark is not available if a straight line was drawn.) OR t2 greater than 2t1 (or similar correct comparison) e.g. ‘not directly proportional as rate increases more than concentration / volume’ 1 1(e)(i) S2O32‒(aq) + 2H+(aq) → S(s) + SO2(aq / g) + H2O(l) 1 1(e)(ii) worse – less thio / FA 3 left in the reaction mixture to react with iodine formed / so time decreased for each run (owtte) e.g. ‘some FA 2 reacts with FA 3 before starting the experiment.’ Allow: same – the concentration of thio / FA 3 is very small so slow reaction with Fe3+ / acid / FA 2 so negligible effect / similar decrease in concentration of thio / FA 3 for each run so effects cancel better – quicker to transfer all the other reactants into the 1st beaker / takes more time to pour from measuring cylinder 1 1(f)(i) Volumes of FA 1, FA 3 and FA 4 are unchanged. 1 FA 2 + water = 20 cm3 Reject if these volumes are unchanged from Experiment 2 values. 1 Question Answer Mark 1(f)(ii) Correctly calculates t = 178 × 10 vol FA 2 Reject if volumes FA 2 + water ≠ 20 cm3 1

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2 When a solution containing thiosulfate ions, S2O32–, is acidified the following reaction occurs. S2O32–(aq) + 2H+(aq) → S(s) + SO2(g) + H2O(l) The solid sulfur that is formed makes the mixture become cloudy. The rate of reaction can then be measured by timing how long it takes for the mixture to become too cloudy to see through. You will investigate how changing the concentration of the thiosulfate ions affects the rate of reaction. Throughout these experiments care must be taken to avoid inhaling the SO2 that is produced. It is very important that as soon as each experiment is complete the contents of the beaker are emptied into the quenching bath. FA 4 is 2.00 mol dm–3 hydrochloric acid, HCl. FA 5 is a solution of sodium thiosulfate, Na2S2O3. distilled water (a) Method Experiment 1 ● Use the 50 cm3 measuring cylinder to transfer 40.0 cm3 of FA 5 into the 100 cm3 beaker. ● Use the 25 cm3 measuring cylinder to measure 20.0 cm3 of FA 4. ● Add the 20.0 cm3 of FA 4 to FA 5 in the beaker and start timing immediately. ● Stir the mixture once and place the beaker on the printed insert. ● View the printed text on the insert from above through the mixture in the beaker. ● Note the time when the print on the insert becomes obscured. ● Record this reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse and dry the beaker so it is ready for use in Experiment 2. Experiment 2 ● Use the 50 cm3 measuring cylinder to transfer 20.0 cm3 of FA 5 into the 100 cm3 beaker. ● Use the 50 cm3 measuring cylinder to transfer 20.0 cm3 of distilled water into the same beaker. ● Use the 25 cm3 measuring cylinder to measure 20.0 cm3 of FA 4. ● Add the 20.0 cm3 of FA 4 to FA 5 in the beaker and start timing immediately. ● Stir the mixture once and place the beaker on the printed insert. ● View the printed text on the insert from above through the mixture in the beaker. ● Note the time when the print on the insert becomes obscured. ● Record this reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse the beaker thoroughly. Keep FA 5 for use in Question 3. Record all your results in a table. You should include the volume of FA 5, the volume of distilled water, the reaction time and the rate of reaction for both experiments. The rate of reaction can be calculated using the following formula. 1000 rate of reaction = reaction time I II III IV [4] (b) A student suggested that the rate of the reaction is directly proportional to the concentration of the thiosulfate ions. State whether your results support this suggestion. Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (c) The student’s suggestion in (b) could be made more reliable by carrying out further experiments. Prepare a table to show three further experiments you could carry out. Show clearly the volumes of FA 4, FA 5 and distilled water that you would use in each of these experiments. Do not suggest a volume of FA 5 that is greater than 40.0 cm3 or less than 20.0 cm3. DO NOT CARRY OUT THESE ADDITIONAL EXPERIMENTS. [2] [Total: 7] 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) Any three observations on heating FA 5: • initially pink crystals • (on gentle heating) solid turns white / paler (pink) • condensation / water droplets / water vapour / misty fumes 1 • (gas) turns (damp blue) litmus red • melts / liquid formed / dissolves • (solid / liquid) turns brown / ochre / yellow-brown 2 • residue is dark brown / black solid 3 1 Allow steam 2 Reject red-brown 3 Reject ppt Ignore bubbles of gas Ignore incorrect positive gas tests 2(a)(ii) FA 6 + H2O2: Effervescence / bubbling / fizzing 1 (Gas / O2) relights glowing splint 1 Question Answer Mark 2(a)(iii) Observations using FA 5(aq): + NaOH: off-white / beige / buff / pale / light brown ppt AND one of: insoluble in excess or darkens on standing / turns brown / darker brown 1 + H2O2: (fizz, etc.) Ignore this box unless there is a ppt + NaOH: dark brown / black ppt Reject if ppt is formed with H2O2 or if ppt dissolves in excess NaOH Allow additional mark for gas relights glowing splint if not awarded in (a)(ii). 1 2(b)(i) Selects for halide: (aqueous) AgNO3 / silver nitrate and (followed by) NH3 / (aqueous) ammonia Ignore preliminary use of nitric acid. 1 Selects for anion containing sulfur: (aqueous) BaCl 2 / Ba(NO3)2 or names and HCl / HNO3 or names Reject if use of sulfuric acid is shown. 1 If neither mark is awarded, allow 1 mark for: AgNO3 / silver nitrate – halide AND BaCl 2 / Ba(NO3)2 (or name) – S-anion Reject if use of sulfuric acid with Ba2+ salt is shown. Question Answer Mark 2(b)(ii) Expected observations: FA 5 FA 7 + Ag+ white ppt * (pale) yellow ppt * + NH3 (ppt) colour darkens / off- white / buff / beige / pale brown * (ppt) insoluble * + Ba2+ no change / no ppt / no reaction / not needed * no change / no ppt / no reaction / not needed * + H+ ignore ignore Two * = 1 mark (round down). Allow 1 mark for the following observations with NH3(aq) if AgNO3(aq) was not selected: FA 5: off-white / beige / buff / pale / light brown ppt AND FA 7: no reaction 3 2(b)(iii) One box = one * Two * = 1 mark (round down). FA 5 FA 7 cation Mn2+ unknown anion Cl ‒ I– Ignore K+ for FA 7. Allow names (manganese(II), unknown, chloride, iodide) for 1 mark. 2 Question Answer Mark 2(b)(iv) FA 5 + Cl 2: no reaction / no (visible) change Allow turns black / dark brown if Mn2+ identified. 1 FA 7 + Cl 2: solution turns yellow / brown or black / dark grey ppt Allow ecf for bromide for either (not both) FA 5 or FA 7: solution turns yellow / red-brown / brown. Allow solution turns orange for either Br – or I−. Allow no reaction / no (visible) change if SO32−/ SO42− identified. 1

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3 (a) FA 6 is a salt containing one cation and one anion. The anion is listed in the Qualitative Analysis Notes. Add all the sample of FA 6 to the 100 cm3 beaker. Dissolve the solid in approximately 50 cm3 of distilled water. Label this solution FA 7. (i) Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of FA 7 in a test‑tube, add a 3 cm depth of aqueous silver nitrate. Pour approximately half the contents of the test-tube into a clean test-tube. Test 2 To the first test-tube add aqueous ammonia. Test 3 To the second test-tube add FA 5, aqueous sodium thiosulfate. [2] (ii) From the results of your tests in (a)(i) suggest which anion is present in FA 6. ....................................................................................................................................... [1] (iii) It is suggested that FA 6 could be sodium sulfite, Na2SO3, or sodium sulfate, Na2SO4. Carry out tests using solution FA 7 in order to decide whether FA 6 is sodium sulfite or sodium sulfate. Record the reagents selected, the results of your tests and your conclusions in the space below. FA 6 is sodium .............................. [2] (iv) Using your conclusion from (a)(iii), write an ionic equation for the reaction between silver nitrate and FA 7. Include state symbols. ....................................................................................................................................... [1] (b) FA 8 is a solution containing one of the cations listed in the Qualitative Analysis Notes. (i) 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 aqueous ammonia until there is no further change, then pour the contents into a boiling tube and add a few drops of aqueous hydrogen peroxide. [3] (ii) Identify the cation in FA 8. cation = .............................. [1] (iii) 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 aqueous potassium iodide, then add FA 5, aqueous sodium thiosulfate. [2] (iv) Explain your observations in (b)(iii). ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] [Total: 14]

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