Cambridge A Level Chemistry 9701 — 2021 Oct/Nov Paper 3 · Variant 6

1 Solid FB 1 is hydrated sodium carbonate, Na2CO3•xH2O. You will determine the value of x in a sample of FB 1. The experiment involves three steps: Step 1 React a known mass of sodium carbonate, FB 1, with an excess of acid. Step 2 Dilute the products of Step 1 to a known volume. Step 3 Carry out a titration to find out how much acid remained after the reaction in Step 1. You will use the results of these three steps to find x. FB 1 is hydrated sodium carbonate, Na2CO3•xH2O. FB 2 is 0.800 mol dm–3 hydrochloric acid, HCl. FB 4 is 0.100 mol dm–3 sodium hydroxide, NaOH. bromophenol blue indicator (a) Method Step 1 ● Label a burette FB 2 and fill this burette with FB 2. ● Run 50.00 cm3 of FB 2 into the 250 cm3 beaker. ● Weigh the container with FB 1. Record the mass. ● Slowly, and in small portions, add FB 1 to the acid. ● Stir the mixture until the fizzing has stopped. Leave the stirring rod in the beaker. ● Reweigh the container with any residue. Record the mass. ● Calculate and record the mass of FB 1 added to the acid. Step 2 ● Stir the mixture from Step 1 and ensure that all the solid has dissolved. Transfer this solution to the graduated flask. ● Rinse the beaker and stirring rod twice with distilled water, then add the washings into the graduated flask. ● Make the solution up to 250 cm3 with distilled water. Thoroughly mix the contents of the flask. This solution is FB 3. Step 3 ● Label the other burette FB 4. Fill this burette with FB 4. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Add several drops of bromophenol blue indicator. ● Perform a rough titration and record your burette readings in the space below. The rough titre is .............................. cm3. ● Carry out as many titrations as you think are necessary to obtain consistent results. ● Make certain any recorded results show the precision of your practical work. ● Record, in a suitable form below, all of your burette readings and the volume of FB 4 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 FB 3 required .............................. cm3 of FB 4. [1] Calculations (c) (i) Give your answers to (c)(ii), (c)(iii), (c)(iv), and (c)(vi) to an appropriate number of significant figures. [1] (ii) Calculate the number of moles of hydrochloric acid in the FB 2 used in Step 1. moles of HCl in FB 2 used in Step 1 = .............................. mol [1] (iii) Use your answer to (b) to calculate the number of moles of sodium hydroxide, FB 4, required to react with 25.0 cm3 of FB 3 in Step 3. moles of NaOH required = .............................. mol Use this answer to deduce the number of moles of hydrochloric acid in 250 cm3 of FB 3. This is the number of moles remaining after the reaction in Step 1. moles of HCl in 250 cm3 of FB 3 = .............................. mol [1] (iv) Use your answers to (c)(ii) and (c)(iii) to calculate the number of moles of hydrochloric acid that reacted with sodium carbonate in FB 1. moles of HCl that reacted with FB 1 = .............................. mol [1] (v) Write an equation for the reaction of sodium carbonate with hydrochloric acid in Step 1. Include state symbols. ....................................................................................................................................... [1] (vi) Use the equation and your answer to (c)(iv) to determine the moles of sodium carbonate present in FB 1. moles of Na2CO3 = .............................. mol [1] (vii) Use your answer to (c)(vi) and your mass of FB 1 to calculate the formula mass of hydrated sodium carbonate. Hence find the value for x. (If you were unable to calculate the number of moles of Na2CO3 in (c)(vi) assume that it is 5.55 × 10–3 mol. This is not the correct value.) x = .............................. [2] (d) State the maximum error in a single balance reading. maximum error in a balance reading = .............................. g Calculate the maximum percentage error in the mass of FB 1 you used. Show your working. maximum percentage error = .............................. % [1] (e) A student decided to use a larger mass of FB 1 when carrying out the same method. What effect would this have on the titre volume in Step 3? Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 19]

Mark scheme: 1(a) I Unambiguous headings 2 balance readings and mass of FB 1 correctly calculated and correctly displayed units 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 + unit • final / end and (burette) reading / volume + unit titre or volume / FB 4 and used / added + unit 1 All accurate burette readings are recorded to the nearest 0.05 cm3 1 The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 1(a) VI, VII and VII: Award V for a range of 15% of the supervisor value for Mr Award VI for a range of 10% of the supervisor value for Mr Award VII for a range of 5% of the supervisor value for Mr If the supervisor titre is ≤ 10.00 cm3 then halve tolerances (0.10, 0.15, 0.25). If supervisor titre & candidate titres ≤ 5.00 cm3 contact TL 3 1(b) Candidate must average two (or more) accurate titres with total spread of not more than 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should be correctly calculated to the nearest 0.01. 1 1(c)(i) All answers in (c)(iii), (c)(iv) and (c)(vi) given to 3 or 4 sf 1 1(c)(ii) Correctly calculates moles of HCl added: 0.800 × (50 / 1000) = 0.0400 1 Question Answer Marks 1(c)(iii) Correctly calculates mol of NaOH used: (b) × (0.1 / 1000) and moles of HCl remaining = moles of NaOH × 10 1 1(c)(iv) Correct expression (d)(iv) = (c)(ii) – (c)(iii) 1 1(c)(v) Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g) OR Na2CO3•xH2O(s) + 2HCl(aq) → 2NaCl(aq) + (x + 1)H2O(l) + CO2(g) 1 1(c)(vi) Correctly uses (c)(iv) / 2 1 1(c)(vii) Correctly uses Mr = m / n = mass of FB 1 from (a) / (c)(vi) 1 Correct expression x = (Mr – 106) / 18 and expressed to the nearest integer or to 2–4 sf 1 1(d) 0.05 or 0.1 if 1 dp balance used / 0.005 or 0.01 if 2 dp balance used / 0.0005 or 0.001 if 3 dp balance used and Correct expression {(2 × error above) / mass from (a)} × 100 1 1(e) Titre would be smaller and More acid has reacted in Step 1 / less acid remaining after reaction in Step 1 1 Question Answer Marks

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2 In this question you will determine the value of y in another sample of hydrated sodium carbonate by thermal decomposition. The equation for the reaction which occurs is given below. Na2CO3•yH2O(s) → Na2CO3(s) + yH2O(g) Solid FB 5 is another sample of hydrated sodium carbonate, Na2CO3•yH2O. (a) Method ● Weigh the empty crucible with its lid. Record the mass. ● Transfer all the FB 5 from its container into the crucible. ● Weigh the crucible, lid and FB 5. Record the mass. ● Calculate and record the mass of FB 5 used. ● Place the crucible and contents on a pipe-clay triangle. ● Heat the crucible gently, with the lid on, for approximately 1 minute. ● Heat strongly, with the lid on, for a further 1 minute. ● Heat strongly, with the lid off, for a further 4 minutes. ● Allow the crucible to cool, with the lid on, for at least 5 minutes. During the cooling period you may wish to start work on Question 3. ● When the crucible is cool, weigh the crucible with its lid and contents. ● Calculate and record the mass of the residue obtained and the mass lost during heating. I II III [3] (b) Use your results to calculate a value for y. y = .............................. [2] (c) Suggest one improvement to the method used in Question 2 which would lead to a more accurate value for y. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 6] 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 Unambiguous headings and correct units: • (mass of) crucible and lid • (mass of) crucible and lid and contents (before heating) / FB 5 • (mass of) FB 5 • (mass of) crucible and lid and residue / contents (after heating) • (mass of) residue / contents (after heating) • (mass of) mass lost during heating / water Units: (g), / g, in grams 1 II All weighings recorded to same number of decimal places and All calculations of mass correct and matching headings 1 III Accuracy (Q) mark • Examiner must check and correct (if necessary) the masses of FB 5 used and of residue (smaller mass) obtained by supervisor and by candidate. • Calculate [(mass of FB 5) / (mass of residue)] for the supervisor to 2 dp • Write this ratio, in a ring, on the candidate script • Calculate [(mass of FB 5) / (mass of residue)] for the Candidate to 2 dp • Calculate d, the difference between these two ratios Award III if d ⩽ 0.20 1 2(b) Correct calculation of moles of: anhydrous sodium carbonate (mass of residue / 106) and water (mass lost / 18) 1 y found by calculation of moles of water / moles of sodium carbonate and answer expressed to nearest integer value or to 2–4 sf 1 2(c) Heat to constant mass / use larger initial mass of FB 5 / use a balance that reads to more dp 1 Question Answer Marks FB 6 is MnO2 FB 7 is FeSO4 FB 8 is NH4Cl

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3 (a) FB 6, FB 7 and FB 8 each contain one cation and one anion. All the cations and anions are different. All the cations and two of the anions are listed in the Qualitative Analysis Notes. FB 7 is an aqueous solution. (i) Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of hydrogen peroxide in a test-tube, add a small spatula measure of FB 6. Test 2 To a 2 cm depth of aqueous potassium manganate(VII) in a test‑tube, add the same depth of aqueous sodium hydroxide. Then add a small spatula measure of FB 6. Stir for about 30 seconds. Filter the mixture and collect the filtrate, then add dilute sulfuric acid to the filtrate. test observations Test 3 To a 1 cm depth of FB 7 in a test‑tube, add an equal volume of hydrogen peroxide. Shake the tube, then add aqueous sodium hydroxide. Test 4 To a 1 cm depth of FB 7 in a test‑tube, add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid. Test 5 Place a small spatula measure of FB 8 into a hard-glass test-tube. Heat the contents. Test 6 Dissolve a small spatula measure of FB 8 in a 2 cm depth of distilled water in a test‑tube. To the solution add a few drops of aqueous silver nitrate, then add aqueous ammonia. [7] (ii) From your test results, identify the anions in FB 6, FB 7 and FB 8. If the tests do not allow you to positively identify an anion, write ‘unknown’. FB 6 FB 7 FB 8 formula of anion [2] (b) Aqueous sodium hydroxide may be used to help identify cations. You will use this reagent to carry out tests on FB 7 and FB 8. Record your method, observations and conclusions in the space below. You are reminded that if any solution is warmed, a boiling tube must be used. I II III IV [4] (c) (i) From your observations, suggest a conclusion that could be made about the chemical behaviour of FB 7 in Test 3 of (a)(i). Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Write an ionic equation for any precipitation reaction you observed in (a)(i). Include state symbols. ....................................................................................................................................... [1] [Total: 15]

Mark scheme: 3 3(a)(i) Test 1 Effervescence * (gas / O2) relights a glowing splint * 1 Test 2 Filtrate / solution is green * filtrate after addition of acid is purple / pink * 1 Test 3 solution turns yellow / yellow-brown * + NaOH brown / red-brown ppt * fizzing (in either part) * 1 Test 4 White ppt * insoluble in acid / no (visible) change * 1 1 Test 5 Solid sublimes / white solid forms further up the test tube / solid disappears / white smoke / white fumes / no residue * Gas / NH3 turns red litmus blue * (On further heating) gas turns blue litmus red * 1 Test 6 White ppt * ppt dissolves * 1 3(a)(ii) FB 6: unknown FB 7: SO42− / sulfate FB 8: Cl − / chloride All three correct: 2 marks any two correct: 1 mark 2 Question Answer Marks 3(b) I Table or clear display of reagents, results and conclusions for minimum of 2 tests attempted 1 II Correct results: FB 7: green ppt and ppt insoluble in excess / turning brown on standing 1 III Heats FB 8 with NaOH and gas / NH3 turns (red) litmus blue 1 IV FB 7 = Fe2+ and FB 8 = NH4+ 1 3(c)(i) FB 7 acts as a reducing agent / FB 7 is oxidised and FB 7 / Fe2+ is oxidised / changes to Fe3+ / red-brown ppt shows Fe3+ / red-brown ppt forms / H2O2 is an oxidising agent / Fe2+ – e− → Fe3+ OR FB 7 acts as a catalyst and rate of decomposition increases 1 3(c)(ii) Any one of: Ag+ (aq) + Cl − (aq) → AgCl (s) Ba2+ (aq) + SO42− (aq) → BaSO4 (s) Fe3+ (aq) + 3OH− (aq) → Fe(OH)3 (s) 1

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