Cambridge A Level Chemistry 9701 — 2021 May/June Paper 3 · Variant 5

1 In this experiment you will carry out a titration to determine the relative formula mass, Mr, of a monoprotic acid, HX. FA 1 is HX, the monoprotic acid. FA 2 is 0.0450 mol dm–3 sodium carbonate, Na2CO3. methyl orange indicator (a) Method Preparing a solution of FA 1 ● Weigh the empty 250 cm3 beaker. Record the mass. ● Transfer all the FA 1 into the beaker. ● Weigh the beaker and FA 1. Record the mass. ● Calculate and record the mass of FA 1 used. ● Add approximately 100 cm3 of distilled water to the FA 1 in the beaker. ● Stir the mixture with a glass rod until all the FA 1 has dissolved. ● Transfer this solution into the 250 cm3 volumetric flask. ● Wash the beaker with distilled water and transfer the washings to the volumetric flask. ● Rinse the glass rod with distilled water and transfer the washings to the volumetric flask. ● Make up the solution in the volumetric flask to the mark using distilled water. ● Shake the flask thoroughly. ● This solution of HX is FA 3. Label the flask FA 3. Results Titration ● Fill the burette with FA 3. ● Pipette 25.0 cm3 of FA 2 into a conical flask. ● Add several drops of methyl orange indicator. ● Perform 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 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 FA 3 added in each accurate titration. I II III IV V VI VII VIII [8] (b) From your accurate titration results, obtain a suitable value for the volume of FA 3 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 required .............................. cm3 of FA 3. [1] (c) Calculations (i) Calculate the number of moles of sodium carbonate present in the volume of FA 2 used in each titration. moles of Na2CO3 = .............................. mol [1] (ii) Give the equation for the reaction of FA 2, Na2CO3, with FA 3, HX. .....Na2CO3 + .....HX → .....NaX + ......................... + ......................... Use your answer to (c)(i) to deduce the number of moles of HX present in the volume you calculated in (b). moles of HX = ...................... mol [1] (iii) Use your answer to (c)(ii) and your data recorded on page 2 to calculate the relative formula mass, Mr, of HX. Show your working. Mr of HX = .............................. [2] (iv) One molecule of HX contains one nitrogen atom, three oxygen atoms, three hydrogen atoms and one atom of another element, E. The identity of E can be found by calculation. Show this calculation and identify E. Element E is .............................. . [1] (d) What is the error in a single reading for the balance that you used? error = .............................. g Calculate the maximum percentage error in the mass of FA 1 that you recorded on page 2. maximum percentage error = .............................. % [1] (e) Suggest and carry out an experiment using aqueous silver nitrate to determine whether the compound AgX is soluble or insoluble in water. method ....................................................................................................................................... .................................................................................................................................................... observations ............................................................................................................................... conclusion .................................................................................................................................. [2] (f) Suggest why the use of methyl orange indicator might give an inaccurate titration result. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 18]

Mark scheme: 1(a) l Unambiguous headings and data are recorded in the space provided • mass of container with FA 1 (do not allow ‘weight’) • mass of (empty) container • mass of FA 1 used 1 II All the following data is recorded • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations 1 III Titre values recorded for accurate titrations, and appropriate headings and units in the accurate titration table • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume used / added / or FA 3 added (not ‘difference’ or ‘total’ or ‘amount’) Allow ‘Vol’, but not V • unit: /cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recordedUnits: (cm3) or /cm3 or in cm3 or cm3 by every entry 1 Question Answer Marks 1(a) IV All accurate burette readings to 0.05 cm3 1 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 For assessment of accuracy (Q) marks, the Examiner should round any burette readings to the nearest 0.05 cm3. Check and correct subtractions. Then select the ‘best’ titres using the hierarchy: • two (or more) accurate identical titres (ignoring any that are labelled ‘rough’), then • two (or more) accurate titres within 0.05 cm3, then • two (or more) accurate titres within 0.10 cm3, etc. These best titres should be used to calculate the mean titre, expressed to nearest 0.01 cm3. Calculate the products of mass and titre for candidate and supervisor to one d.p, as shown below. • Product = ‘best’ mean titre × mass of FA 1 used. (Theoretical product value = 56.0 cm3 g) If no weighings are discernible, assume mass of FA 1 = 2.25 g Calculate the difference (δ) between the candidate’s product and the supervisor’s product. Award the accuracy (Q) marks as shown below. Award accuracy marks as follows: VI Award if δ ⩽ 2.0 cm3 g VII Award if δ ⩽ 1.0 cm3 g VIII Award if δ ⩽ 0.5 cm3g (3) 8 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) Correctly calculates (to 3 or 4 sig fig) No of moles of Na2CO3 used = 0.00113 or 0.001125 1 Question Answer Marks 1(c)(ii) Correct equation AND calculates moles of HX • Na2CO3 + 2HX → 2NaX + CO2 + H2O and • No of moles of HX = 0.00226(0) or 0.00225(0) 1 1(c)(iii) Correct expression shown for scale up M1: No of moles HX in 250 cm3 = (c)(ii) × 250 / mean titre Correctly uses expression for Mr M2: Mr = mass of FA 1 used/moles of FA 1 in 250 cm3 and answer given to 3 or 4 sf and to the correct power of 10 2 1(c)(iv) Appropriate identity of E Ar = answer (iii) − 14 − 3 − 48 (= (iii) – 65) and E chosen with Ar closest to this value. 1 1(d) Correct expression for % error = (2 × balance error/mass of FA1 used) x 100 1 1(e) M1: Method Add (aqueous silver nitrate to) FA 3 or to a solution of FA 1 M2: Observation and conclusion No reaction / no change / no precipitate / colourless solution (formed) and AgX is soluble 2 1(f) Accept one answer from • end-point colour change is indistinct / difficult to see • colour change occurs over addition of significant volume • accept an answer based on pH curve – e.g. colour change range not within ‘vertical’ part of graph 1

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2 You will determine the percentage by mass of water of crystallisation in a hydrated salt by thermal decomposition. When a hydrated salt is heated it loses its water of crystallisation. FA 4 is the hydrated salt. (a) Method ● Weigh the crucible with its lid. Record the mass. ● Transfer 2.0–2.2 g of FA 4 from the container into the crucible. ● Weigh the crucible, lid and FA 4. Record the mass. ● Calculate and record the mass of FA 4 used. ● Place the crucible and contents on the pipe-clay triangle. ● Heat gently, with the lid on, for approximately 1 minute. ● Heat strongly, with the lid off, for a further 4 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. During the cooling period, you may wish to begin work on Question 3. ● When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. ● Heat strongly, with the lid off, for a further 2 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. ● When it is cool, weigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass lost. Keep FA 4 for use in Question 3. Results I II III IV V [5] (b) (i) Calculate the percentage by mass of water of crystallisation in FA 4. percentage by mass of H2O in FA 4 = .............................. % [1] (ii) State what assumption you made about anhydrous FA 4 in your calculation in (b)(i). ............................................................................................................................................. ....................................................................................................................................... [1] [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 Unambiguous headings and correct units for four weighings • Mass of crucible, lid (empty) • Mass of crucible, lid and FA 4 (or ‘contents before heating’) • Mass of crucible, lid and contents after heating / residue (1) • Mass of crucible, lid and contents after heating / residue (2) II Readings are appropriately recorded • All weighings shown are recorded to same decimal places (one or more). • Mass of FA 4 is within range 2.00–2.20 g (from weighings) Third and fourth weighings are within 0.04 g of each other (or identical if a 1 dp balance was used) 1 III Correct subtractions (answers given to 2 or more sig fig) and Mass of FA 4 used and mass of water lost both listed. 1 Accuracy marks Use the theoretical mass ratio water/FA 4 = 0.147 • Examiner must check and correct (if needed) the mass of FA 4 used and water removed (after re-heating) by the candidate. • Work out ratio mass of water/mass of FA 4 for the candidate (to 3 dp) Award accuracy marks as follows: IV Award if ratio in range 0.137–0.157 V Award if ratio in range 0.142–0.152 (2) 5 2(b)(i) Correctly calculates percentage by mass of water Answer = ratio (see above) × 100 1 2(b)(ii) Assumption The residue does not decompose when heated / the mass loss is only due to water (OWTTE) 1

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3 (a) FA 4 contains one cation and one anion both of which are listed in the Qualitative Analysis Notes. Dissolve the remainder of the FA 4 in approximately 20 cm3 of water in the 100 cm3 beaker. Use a 1 cm depth of this solution in a test-tube for each of the tests. (i) The anion in FA 4 is chloride. State which reagents you would use to identify this ion conclusively. ............................................................................................................................................. Use your reagents to test your solution of FA 4. Record your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. In what way are your observations not consistent with the expected results for chloride? ............................................................................................................................................. ............................................................................................................................................. [3] (ii) Add an equal volume of dilute sulfuric acid to aqueous FA 4. Record your observations and deduce the identity of the cation in FA 4. observations ........................................................................................................................ identity of cation .................................................................................................................. Give the ionic equation for this reaction. Include state symbols. ............................................................................................................................................. [2] (b) Use the conclusions in (a)(i) and (a)(ii) to calculate the relative formula mass of anhydrous FA 4. Mr = .............................. Use your results from Question 2 to calculate the number of moles of water of crystallisation in one mole of hydrated FA 4. moles of H2O = .............................. [3] (c) FA 5 contains one cation and one anion, both of which are listed in the Qualitative Analysis Notes. (i) Heat a small spatula measure of FA 5 in a hard-glass test-tube. Keep heating until no further change is observed. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) To a 1 cm depth of dilute sulfuric acid in a test-tube, add a small spatula measure of FA 5. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [3] (iii) Place a small spatula measure of FA 5 into a boiling tube. Add a 1 cm depth of aqueous sodium hydroxide and warm the mixture gently. Record your observations. ............................................................................................................................................. ....................................................................................................................................... [1] (iv) From the results of your tests, deduce the chemical formula of FA 5. FA 5 is ................................................... . [1] [Total: 15]

Mark scheme: 3(a)(i) Correct reagents M1: (Reagents needed are) silver nitrate and ammonia M2: White ppt with AgNO3 and Precipitate is partially / mostly soluble in (excess) ammonia. M3: the precipitate (of silver chloride) should dissolve completely in excess ammonia/ the ppt is insoluble in (excess) ammonia 3(a)(ii) M1: White precipitate / solid and cation is barium M2: Ionic equation Ba2+(aq) + SO42–(aq) → BaSO4(s) 2 Question Answer Marks 3(b) Calculation of no of moles of water M1: Mr of anhydrous salt = 208.3 M2: n(anhy salt : water) =mass residue : mass water Mr 18 = ans : ans M3: ratio of moles and answer given to integer value or up to 4 sf OR M2: Candidate shows valid working to calculate Mr of hydrated salt or working to calculate mass of water from 1 mole FA 4. (e.g. Mr (hydrated salt) = 208.3 × mass FA 4 used / mass residue) or mass water (in 1 mol FA 4) = 208.3 × mass water lost / mass of residue M3 Candidate attempts to divide mass of water by 18 and expresses moles of water as an integer or up to 4 sf 3 3(c)(i) Heating FA 5: there are four marking points * FA 5 is a white solid / powder / crystals * Condensation / steam / water vapour forms * (Gas / vapour / ammonia) turns (red) litmus blue * No residue / solid left (in the tube) or solid disappears / sublimes 3 or 4 asterisks (*) = 2 marks 2 asterisks (*) = 1 mark 2 Question Answer Marks 3(c)(ii) FA 5 + acid: there are 6 marking points *Fizzing / bubbling / effervescence (but not ‘gas produced’) *Rapid / vigorous (reaction) owtte *Solid dissolves / (colourless) solution formed *Reaction is endothermic / solution gets colder *Attempts to test with limewater *Gas / carbon dioxide turns limewater milky / white precipitate ‘cloudy’ is not accepted, but ‘cloudy white’ is OK. 2 asterisks (*) = 1 mark (round down) 3 3(c)(iii) gas / ammonia turns (red) litmus blue. 1 3(c)(iv) Conclusion FA 5 is (NH4)2CO3 1

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