Cambridge A Level Chemistry 9701 — 2021 Feb/March Paper 3 · Variant 3

1 In this experiment you will carry out a titration to identify the Group 1 metal, M, present in a metal hydrogencarbonate, MHCO3. FA 1 is 0.0550 mol dm–3 sulfuric acid, H2SO4. FA 2 is the metal hydrogencarbonate, MHCO3. bromophenol blue indicator (a) Method Preparing a solution of FA 2 ● Weigh the stoppered container of FA 2. Record the mass in the space below. ● Tip all the FA 2 into the beaker. ● Reweigh the container with its stopper. Record the mass. ● Calculate and record the mass of FA 2 used. ● Add approximately 100 cm3 of distilled water to FA 2 in the beaker. ● Stir the mixture with a glass rod until all the FA 2 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 MHCO3 is FA 3. Label the flask FA 3. Titration ● Fill the burette with FA 1. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add a few drops of bromophenol blue indicator to the conical flask. ● 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 1 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 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required .............................. cm3 of FA 1. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of sulfuric acid present in the volume of FA 1 calculated in (b). moles of H2SO4 = .............................. mol [1] (iii) Complete the equation for the reaction of sulfuric acid and MHCO3. State symbols are not required. .....MHCO3 + .....H2SO4 → .....M2SO4 + .................... + .................... Use your answer to (c)(ii) to deduce the number of moles of MHCO3 used in each titration. moles of MHCO3 = .............................. mol [1] (iv) Use your answer to (c)(iii) and your data on page 2 to calculate the relative formula mass, Mr, of MHCO3. Mr of MHCO3 = .............................. [1] (v) Calculate the relative atomic mass, Ar, of M. Ar of M = .............................. Suggest the identity of M. M is .............................. . [1] (d) (i) A student used a pipette that was labelled 25.0 ± 0.06 cm3 to measure FA 3. Show how you calculate the maximum percentage error in the volume of FA 3. [1] (ii) The student suggested that it would have been more accurate to measure the volume of FA 3 with a burette instead of the pipette. State and explain whether you agree with the student. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 16]

Mark scheme: 1(a) I Headings and data are recorded in the space provided • (mass of) container with FA 2 • (mass of empty) container • (mass of) FA 2 (used) Subtraction for the mass of FA 2 used must be correct Headings must be unambiguous and include either ‘mass’ or g for each piece of datum. Reject ‘weight’. II The following data must be shown: • 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 correct headings and units in the accurate titration table • initial/start and (burette) reading / volume • final/end and (burette) reading / volume • titre or volume / FA 1 and used / added reject ‘difference’ or ‘total’ or ‘amount’ or ‘V’ but allow ‘vol’ • unit: / cm3 or (cm3) or in cm3 for each heading or cm3 unit given for each volume recorded 1 IV All accurate burette readings are recorded to the nearest 0.05 cm3, including 0.00. Reject 50(.00) as an initial burette reading Reject if more than one final burette reading is 50.(00) Reject any burette reading is greater than 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 is recorded as an integer (apart from an initial 0 cm3) 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 mass of FA 2 to 2 dp. Examiner calculates candidate’s corrected average titre / candidate’s mass of FA 2 to 2 dp. Subtract the candidate value from that of the supervisor: δ Question Answer Marks 1(a) Award VI if 0.40 < δ ⩽ 0.60 cm3 g–1 1 Award VI and VII if 0.20 < δ ⩽ 0.40 cm3 g–1 1 Award VI, VII and VIII if δ ⩽ 0.20 cm3 g–1 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 1(b) Candidate calculates the mean correctly: • Candidate must take the average of two (or more) accurate 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) All answers given to (c)(ii) – (c)(v) must be to 3 or 4 sig fig (Minimum 3 answers required to award the mark) 1 1(c)(ii) Correctly calculates: no of moles of H2SO4 used = 0.0550 × mean titre/1000 The candidate’s mean titre must be used. 1 1(c)(iii) Correct equation and correctly uses (ii) • 2MHCO3 + H2SO4 → M2SO4 + 2CO2 + 2H2O Allow multiples and ignore state symbols. AND • no of moles of MHCO3 = 2 × answer (ii) 1 1(c)(iv) Correctly uses (iii) Mr = mass of FA 2 used/10 x answer (iii) 1 Question Answer Marks 1(c)(v) Correct use of Mr and appropriate identity of M • Ar = answer (iv) – 61 AND • M identified as Group 1 metal with closest Ar Li 0–14.9; Na 15.0–31.0; K 31.1–62.2; Rb 62.3–109.1; Cs 109.2–250 Reject if the Ar calculated is > 250 or if Ar < 0 1 1(d)(i) Correct expression % error = 0.06/25 × 100 (= 0.24 %) No answer needed but reject incorrect answer. No mark for just 0.24 without some working. 1 1(d)(ii) Student is incorrect AND error in burette reading = 2 × 0.05 > 0.06 (or candidate compares the % errors, 0.40 % and 0.24 %) Reject suggestion that error in 1 burette reading is 0.1 1

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2 In this experiment you will determine the relative formula mass of the same metal hydrogencarbonate, MHCO3, by thermal decomposition. Then you will compare the result obtained with your answer from 1(c)(iv). FA 4 is another sample of the metal hydrogencarbonate, MHCO3. (a) Method ● Weigh the empty crucible with its lid. Record the mass. ● Transfer all the 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 a pipe-clay triangle. ● Heat the crucible gently, with the lid on, for approximately one minute. ● Heat strongly, with the lid off, for a further four minutes. ● Replace the lid and leave the crucible to cool for at least five minutes. During each cooling period, you may wish to work on Question 3. ● When the crucible has cooled, weigh the crucible with its lid and contents. Record the mass. ● Heat strongly, with the lid off, for a further two minutes. ● Replace the lid and leave the crucible to cool for at least five minutes. ● When the crucible has cooled, reweigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass of residue obtained. ● This residue is FA 5. Keep FA 5 for use in 2(b)(i). Results I II III IV V [5] (b) (i) Pour a 1 cm depth of dilute hydrochloric acid into a test‑tube. Add a spatula measure of residue FA 5 to the acid. Record all your observations and identify any gas formed. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Use your observations in (b)(i) to identify the anion in FA 5. Assume all the MHCO3 has decomposed. Anion in FA 5 is .............................. . [1] (iii) Steam is produced when the metal hydrogencarbonate, FA 4, is thermally decomposed. Use your answer in (b)(ii) to complete the equation for the thermal decomposition of MHCO3. Include state symbols. .....MHCO3(s) → .....CO2(g) + ...................... + ...................... [1] (iv) The number of moles of carbon dioxide given off during the thermal decomposition is given by the formula below. mass lost during heating moles of CO2 = (Mr of CO2 + Mr of H2O) Calculate the number of moles of carbon dioxide given off. moles CO2 = .............................. mol [1] (v) Calculate the relative formula mass, Mr, of MHCO3. Show how you obtained your answer using your data from Question 2. Mr of MHCO3 = .............................. [1] (vi) You have obtained two values for the Mr of MHCO3; one in 1(c)(iv) and another in 2(b)(v). State which value is likely to be more accurate. Explain your answer in terms of the practical procedures used. The Mr obtained in Question ................... is more accurate. reason ................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 12] 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 of data: 6 correct headings shown • Mass of crucible (and lid) • Mass of crucible (lid) and FA 4 (or ‘contents before heating’) • Mass of crucible (lid) and contents/residue after heating (1) • Mass of crucible (lid) and contents/residue after heating (2) • Mass of FA 4 (used) • Mass of FA 5 / residue obtained (may be two readings) Reject FA 4 in 3rd and 4th readings. Lid can be omitted (provided this is done consistently for all readings). 1 II Readings recorded appropriately • Unit given for all data quoted. All four weighings needed. Units: / g or (g) or in gram(me)s in general mass heading or after every entry / heading • All weighings recorded to same number of decimal places (one or more). • Third and fourth weighings are within 0.04 g of each other (or both equal if a one decimal place balance was used) Reject if any readings are impossible (e.g. 4th weighing lower than 1st or 4th greater than 2nd) 1 III Correct subtractions for masses Mass of FA 4 used and residue / FA 5 obtained are correctly shown. 1 IV and V: Accuracy (Q) marks • For assessment of accuracy, examiner must check and correct (if necessary) the masses of FA 4 used and of residue obtained by the candidate. • Write and ring the theoretical ratio of 1.58 on the candidate’s script. • Calculate ratio (mass FA 4/mass of residue) for candidate’s results to 2 dp • Calculate δ, the difference between these two ratios. Question Answer Marks 2(a) Award IV only if 0.05 < δ ⩽ 0.10 1 Award IV and V if δ ⩽ 0.05 1 2(b)(i) 2 or 3 bullets = 1 mark 4 bullets = 2 marks • fizzing / bubbling / effervescence • gas turns lime water milky / white ppt / cloudy white Reject ‘cloudy’ alone • gas is identified as carbon dioxide Reject if an incorrect positive gas test is observed • reaction is rapid / vigorous or colourless solution forms / (all) solid dissolves 2 2(b)(ii) CO32– / carbonate 1 2(b)(iii) 2MHCO3(s) → CO2(g) + H2O(g or l) + M2CO3(s) Allow use of the symbol of Group 1 metal identified in question 1 1 2(b)(iv) Correctly calculates no of moles of CO2 n(CO2) = mass loss during heating/62 Answer must be given correct to 2–4 sig figs Allow ecf from (incorrect) subtraction mass shown in list (FA 4 – residue / FA 5) 1 2(b)(v) Correct use of (iv) to calculate Mr Mr = mass of FA 4 used/2 × answer (iv) Answer to 2 – 4 sf but do not penalise sf more than once in Q2. Ecf on incorrect mole ratio in equation 1 Question Answer Marks 2(b)(vi) Any one answer from the following: • 2 is more accurate since fewer readings required. • 2 is more accurate since there is less cumulative error / fewer processes involved Reject less total error • 1 is more accurate because 2 was not heated to constant mass. Candidate must refer to their results Reject if mass readings after heating are within 0.02 of each other • 1 is more accurate since titres were consistent / concordant. Ignore comments about burettes/pipettes being more accurate 1

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3 (a) FA 6 contains one cation and one anion both of which are listed in the Qualitative analysis notes. (i) Heat FA 6 gently for one minute in the hard‑glass test‑tube in which it is supplied. Then heat strongly until no further change occurs. Record all of your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Identify the ion that must be present in FA 6. ....................................................................................................................................... [1] (b) (i) FA 7 and FA 8 are aqueous solutions. Each solution contains one cation and one anion both of which are listed in the Qualitative analysis notes. Use 1 cm depths of FA 7 or FA 8 in test‑tubes for the following tests. Complete the table by recording your observations. observations test FA 7 FA 8 Test 1 Add a few drops of aqueous acidified potassium manganate(VII), then add a few drops of starch indicator. Test 2 Add a few drops of aqueous silver nitrate, then add aqueous ammonia. Test 3 Add aqueous sodium hydroxide, then pour the mixture into a boiling tube. Warm gently and carefully, then add a piece of aluminium foil. Test 4 Add a few drops of dilute sulfuric acid. [6] (ii) Deduce the chemical formulae of FA 7 and FA 8. FA 7 is ...................................................... and FA 8 is ..................................................... . [2] (iii) Give the ionic equation for the reaction of FA 8 with sulfuric acid. Include state symbols. ....................................................................................................................................... [1] [Total: 12] Qualitative analysis notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. pale blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Mark scheme: 3(a)(i) 1 mark for correct gas test: (gas / vapour / fumes) turn (moist red) litmus to blue Reject if incorrect gas identified 1 mark for any two bulleted observations correct: • solid sublimes / white solid forms near top of tube (owtte; allow residue for solid) Reject ‘solid evaporated’ Allow ‘white layer formed around glass tube’ (bod) • white and smoke / vapour / fumes produced Reject ‘effervescence’ Reject ‘gas’ • no residue (at bottom of tube) Allow ‘crystals disappear completely’ • after heating for some time, gas turns (moist blue) litmus to red 2 3(a)(ii) ammonium / NH4+ Reject if more than one ion identified 1 Question Answer Marks 3(b)(i) 14 observations. Two * = 1 mark (round down) Reject ‘no observation’ (for ‘no change’) the first time seen, then allow 6 test reagent FA 7 FA 8 1 KMnO4 Solution / turns and yellow / red-brown / orange-brown / brown * Reject any reference to purple colour at end Reject ppt Ignore starch black / dark blue / blue-black / black-purple * Reject purple on its own Ignore 2 AgNO3 (pale) yellow precipitate (formed) * Reject creamish-yellow white precipitate (formed) * Reject off-white NH3 ppt does not dissolve / insoluble / no change * ppt (mostly) dissolves or partially dissolves or (slightly) cloudy mixture forms or some white ppt remains * Reject ‘clear solution’ 3 NaOH (cold) no reaction / no change / no precipitate * Allow ‘no visible observation’ white precipitate and insoluble in excess * Reject any variation on white, e.g. off-white NaOH (hot) gas / NH3 turns (red) litmus to blue * Ignore observations when heated (but reject litmus goes blue at any stage of this test) Al Ignore observation(s) with Al fizzing / bubbling / effervescence or gas / H2 pops with lighted spill * 4 H2SO4 no change / no reaction or solution remains colourless * Reject ‘no ppt’ white precipitate (formed) * Question Answer Marks 3(b)(ii) FA 7 is NH4I FA 8 is BaCl2 ALLOW CaCl2 / CaBr2 / BaBr2 for FA8 If both are named correctly award one mark (out of 2). If both cations are correct, award one mark (out of 2). 2 3(b)(iii) Ba2+(aq) + SO42–(aq) → BaSO4(s) State symbols are required. Allow ecf for Ca2+ or Mg2+ in (ii) 1

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