Cambridge A Level Chemistry 9701 — 2020 Oct/Nov Paper 3 · Variant 5

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 Clearly shows initial and final mass and both recorded to the same precision and with correct units. Units: (g) or / g or in g or g by every entry (including mass used if given) Reject weight 1 II The following data must be shown • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings 1 III Headings and units correct for accurate titration table and headings match readings. • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FA 2 and used / added Units: (cm3) or / cm3 or in cm3 or cm3 by every entry Allow vol for volume, value for reading Allow change in volume Reject difference, total or amount 1 IV All accurate burette readings are recorded to 0.05 cm3 (including 0.00) Reject if 50(.00) is used as an initial burette reading; Reject if more than one final burette reading is 50(.00) Reject if any burette reading is > 50(.00) 1 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. Ignore any titre labelled ‘rough’ Reject if any ‘accurate’ burette reading (apart from an initial 0 cm3) is given to zero dp. 1 Check and correct titre and mass 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 calculates supervisor’s corrected average titre × supervisor’s corrected mass to 1 dp. Examiner calculates candidate’s corrected average titre × candidate’s corrected mass to 1 dp. Subtract the candidate product value from that of the supervisor: δ Question Answer Marks 1(a) Award VI if 4.0 < δ ⩽ 6.0 g cm3 1 Award VI and VII if 2.0 < δ ⩽ 4.0 g cm3 1 Award VI, VII and VIII if δ ⩽ 2.0 g cm3 1 If there is only one accurate titration award accuracy marks based on that titration without further penalty. If only a rough titration is shown award accuracy marks based on this value but cancel one accuracy mark. Apply spread penalty as follows:if titres selected (by examiner) differ ⩾ 1.00 cm3 then cancel one accuracy mark. If Supervisor’s value ⩽ 10.00 cm3 then halve tolerances (3 marks: δ ⩽ 1.0; 2 marks: δ ⩽ 2.; 1 mark: δ ⩽ 3.0 g cm3) 1(b) Candidate must average two (or more) titres that are all within 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1 1(c)(i) Answers for (ii) and both parts of (iii) are quoted to 3−4 sf. 1 1(c)(ii) Correctly calculates 1.25 × 10–3 1 1(c)(iii) Correctly uses 2.50 × 10−3 AND 2.50 × 10−3 × ( ) ( )   =       250 0.625 b b 1 1(c)(iv) Correctly uses correctly calculated mass from ( ) ( ) ans a iii 1 Display of ( ) − ANS 158.2 18 1 1(c)(iv) Uses values to calculate x to the nearest integer 1 Question Answer Marks 1(d)(i) Uncertainty in a single reading: for 1 dp balance allow the uncertainty given to be ± 0.1 or 0.05 for 2 dp balance allow the uncertainty given to be ± 0.01 or 0.005, etc. AND Display of   × ×     2 uncertainty given 100 candidate mass 1 1(d)(ii) Correctly uses ( )( ) ( )   −  ×     100 100 d i candidate’s Mr OR ( ) ( ) ( ) ( )( ) − × mass 2 single uncertainty ans a c iii Allow numerator as (mass (a) – single uncertainty) if lack of doubling already penalised in (d)(i). 1 1(e) Titre value is less because concentration of thiosulfate is greater 1 1(f) The indicator colour change is easier to see blue-black to colourless. OR The dark colour of the aqueous iodine makes the burette harder to read. 1

More questions on Reacting masses and volumes (of solutions and gases)

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 Appropriate headings and units for data given. • volume of FA 5 • volume of water • time • rate Volumes in cm3 or / cm3 or (cm3). Time in seconds or / s or (s), rate in s–1 or (s-1). Ignore 1000. Allow ecf for display of units if penalised in 1(a). II Both times recorded to the nearest second. 1 Examiner corrects times to the nearest second. Examiner calculates ratio: time for experiment 2 time for experiment 1 to 1 dp. III Award if this ratio is between 1.9 and 2.1. 1 IV Both rates correctly calculated using 1000 time and recorded to a minimum of 2 sf. 1 2(b) Agree because ratio is almost 2. OR Disagree because ratio is not 2. Allow answers in terms of time 1 2(c) Records volumes of FA 4, FA 5 and water. 3 additional volumes of FA 5 chosen with intervals not less than 4.0 cm3 and all volumes of FA 5 ⩾ 24.0 cm3 and ⩽ 36.0 cm3 1 In all 3 additional experiments water is added to make a total volume of FA 5 + water = 40.0 cm3 Allow for intervals of less than 4.0 cm3 for FA 5. Allow if FA 4 omitted. Reject if FA 4 ≠ 20(.0) cm3 1

More questions on Rate of reaction

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]

Mark scheme: 3(a)(i) + AgNO3 gives a white ppt 1 soluble in both NH3(aq) and FA 5 / thio 1 3(a)(ii) Cl − / chloride 1 3(a)(iii) Selects BaCl 2 OR Ba(NO3)2 and HCl OR HNO3 OR selects acidified (aqueous) KMnO4 OR add named mineral acid and test for SO2 (e.g. blue litmus turns red; acidified aqueous manganate(VII) paper turns colourless) 1 Clear display of results to show: white ppt and (partially) soluble in acid OR KMnO4 decolourises OR positive result for SO2 AND FA 6 = sodium sulfite 1 3(a)(iv) 2Ag+(aq) + SO32−(aq) → Ag2 SO3(s) Allow sulfate if ppt seen in (iii): 2Ag+(aq) + SO42−(aq) → Ag2 SO4(s)(sic) 1 3(b)(i) (Pale) blue ppt dissolves in excess to give a dark blue solution. 1 + H2O2 solution turns black / dark green OR black / dark green solid produced AND Effervescence / fizzing / bubbling 1 gas / oxygen relights a glowing splint 1 3(b)(ii) Cu2+ / copper(II) 1 Question Answer Marks 3(b)(iii) + KI(aq) (turns) brown / yellow-brown / orange-brown / grey-brown Ignore state Allow mustard (brown) Reject red-brown 1 + FA 5 then (brown solution becomes paler) ppt is off-white / white Allow cream / pale grey ppt. Ignore effect of excess thio / FA 5. 1 3(b)(iv) Any 2 of: • (mixture) turns brown owing to the production of iodine • ppt formed is copper(I) iodide (allow copper(II) iodide) • I– is oxidised (by Cu2+) OR Cu2+ is reduced (by I–) I2 is reduced (by S2O32–) OR S is oxidised (by I2) (ignore oxidation state of S) 2

More questions on General characteristic chemical properties of the first set of transition elements, titanium to copper