Cambridge A Level Chemistry 9701 — 2022 May/June Paper 3 · Variant 3

1 Acids donate protons, H+, in aqueous solution. The number of moles of H+ donated per mole of acid is the proticity of the acid. In this experiment, you will carry out a titration to determine the proticity of phosphoric acid, H3PO4, when it reacts with sodium hydroxide, NaOH. FA 1 is aqueous phosphoric acid, containing 6.86 g dm–3 H3PO4. FA 2 is 0.150 mol dm–3 sodium hydroxide, NaOH. FA 3 is thymolphthalein indicator. (a) Method ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 1 into a conical flask. ● Add a few drops of FA 3. ● 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 your burette readings and the volume of FA 2 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtained the mean value. 25.0 cm3 of FA 1 required .............................. cm3 of FA 2. [1] (c) Calculations (i) Calculate the amount, in mol, of sodium hydroxide present in the volume of FA 2 calculated in (b). amount of NaOH = .............................. mol [1] (ii) Use the information on page 2 to calculate the amount, in mol, of phosphoric acid present in 25.0 cm3 of FA 1. amount of H3PO4 = .............................. mol [1] (iii) Deduce whether phosphoric acid behaves as a monoprotic, diprotic or triprotic acid in this titration. Explain your reasoning. H3PO4 is a .............protic acid. explanation ............................................................................................................................................. ............................................................................................................................................. [1] (iv) Give the equation for this reaction of phosphoric acid, H3PO4, with sodium hydroxide. ....................................................................................................................................... [1] (d) (i) A student uses a pipette that is labelled 25.0 ± 0.06 cm3 to measure FA 1. Calculate the maximum percentage error in the volume of FA 1. Show your working. maximum percentage error = ..............................% [1] (ii) The student suggests it would be more accurate to measure the volume of FA 1 with a burette instead of the pipette. State whether you agree with the student. Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 14]

Mark scheme: 1(a) I All the following data are recorded:  two burette readings AND titre for the rough titration  initial and final burette readings for two (or more) accurate titrations 7 II Correct headings and units in the accurate titration table  initial / start AND (burette) reading/volume + unit  final / end AND (burette) reading/volume + unit  titre + unit OR volume / FA 2 and used / added + unit III All accurate burette readings are recorded to the nearest 0.05 cm3 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre For assessment of accuracy marks, round all 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. Write the Supervisor’s [corrected] mean titre in a ring on each candidate script Calculate the difference () between the candidate’s mean titre and the supervisor’s. Write the value of  on each script. Award the accuracy marks as shown below. Award V if  ⩽ 0.50 (cm3) Award VI if  ⩽ 0.30 Award VII if  ⩽ 0.20 1(b) Correctly calculates the mean titre correct to 2 dp from accurate titres that are within 0.20 cm3 total spread 1 1(c)(i) Correctly calculates amount of NaOH used = 0.150  1000 (b) AND answer to 3 or 4 significant figures 1 Question Answer Marks 1(c)(ii) Correctly calculates amount of H3PO4 in 25 cm3 = 6.86 1 40 98.0  = 0.00175(0) = 1.75  10−3 mol 1 1(c)(iii) Acid is diprotic AND reason using answers (c)(i) and (c)(ii) Explanation: mole ratio of ans (c)(i) : ans (c)(ii) = 2 : 1 (approx) OR calculation shown 1 1(c)(iv) Equation follows from the proticity stated in (c)(iii) Expected equation: H3PO4 + 2NaOH → Na2HPO4 + 2H2O 1 1(d)(i) Correct expression for % error = 100 0.06 25  (= 0.24%) 1 1(d)(ii) Student is incorrect AND explanation given: burette error = 2  0.05 (which is >0.06 / so 0.40% > 0.24%) 1

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2 In this experiment you will identify the metal, M, in a metal carbonate, MCO3, by thermal decomposition. MCO3(s) → MO(s) + CO2(g) FA 4 is the metal carbonate, MCO3. (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 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 has cooled, 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 the crucible has cooled, reweigh the crucible with its lid and contents. Record the mass. ● Calculate and record the total loss of mass and the mass of residue obtained. ● This residue is FA 5. Keep FA 5 for use in 2(d). Results I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide given off in your experiment. amount of CO2 = .............................. mol [1] (ii) Calculate the relative formula mass, Mr, of MCO3. Mr of MCO3 = .............................. [1] (iii) From your results, deduce the identity of M. Show your reasoning. M is = .............................. [1] (c) A student carries out the same procedure, using the same mass of solid. However, the student uses the basic carbonate, MCO3•M(OH)2, instead of the pure carbonate, MCO3. When the metal hydroxide part of the basic carbonate decomposes, metal oxide and steam are produced. The metal carbonate part decomposes in the usual way. State how the loss of mass from the student’s solid compares with the loss of mass you obtained when you carried out your experiment. Explain your reasoning. .................................................................................................................................................... .............................................................................................................................................. [2] (d) Use a spatula to transfer a small quantity of your cold residue, FA 5, into a test-tube. Add about a 1 cm depth of dilute hydrochloric acid to the FA 5 in the test-tube. Record what you observe. .................................................................................................................................................... .................................................................................................................................................... State whether or not the thermal decomposition of MCO3 is complete. Justify your answer based on your observations. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... [2] [Total: 12] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: ● colour changes seen ● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added ● the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a) I Unambiguous headings and units for four weighings and entered in the space provided:  (mass of) crucible, lid (empty)  (mass of) crucible, lid and FA 4 (or ‘contents before heating’)  (mass of) crucible, lid and FA 5 / residue / contents after first heating  (mass of) crucible, lid and FA 5 / residue / contents after second heating Units: / g or (g) or gram(me)s or in grams II Readings are appropriately recorded:  all weighings recorded to same number of decimal places (two or more)  mass of FA 4 is within the range 0.80–1.80 g (from weighings)  fourth weighing is within +0.02 and –0.05 g of third weighing III Correct subtractions to give:  mass of FA 4  mass of FA 5  mass loss For assessment of accuracy marks: calculate supervisor’s mass ratio (to 2 d.p.) = mass mass FA 4 FA 5 (residue) Write this value in a ring on each script. Calculate the 25% and 10% ranges correct to 2 dp. Calculate the candidate’s mass ratio (to 2 dp) = mass mass FA 4 FA 5 (residue) Award accuracy marks as shown below. Award IV if  is within 25% of supervisor AND ratio > 1.00 Award V if  is within 10% of supervisor AND ratio > 1.00 2(b)(i) Correctly calculated amount of CO2 = mass loss 44 mol AND answer to 2–4 significant figures 1 Question Answer Marks 2(b)(ii) Correctly uses Mr of FA 4 = mass of used FA 4 (b)(i) AND answer to 2–4 sf 1 2(b)(iii) Ar of metal = (b)(ii) – 60 AND Group 2 metal correctly deduced from Ar Be if Ar is in range 0 – 12.1: Mg for Ar 12.2 – 32.2: Ca for Ar 32.2 - 63.8: Sr for Ar 63.9 – 112.4: Ba for Ar 112.5 – 250 (Other metals forming white carbonates with formula MCO3 are acceptable provided the Ar is close to the value given in the periodic table.) 1 2(c) M1: student’s mass loss is lower AND Mr H2O < Mr CO2 (or values given) M2: using Mg as example % loss from Mg(OH)2 = 18 58.3        100 % loss from MgCO3 = 44 84.3        100 (which is greater than % for Mg(OH)2) 2 2(d) EITHER M1: solid dissolves OR colourless solution formed M2: decomposition is complete AND reason: no gas / no CO2 / no fizzing produced (when acid added) OR M1: effervescence (owtte) OR gas / CO2 gives white ppt with limewater M2: decomposition is not complete AND reason: carbonate reacts / fizzes / gives out CO2 with acid / oxide does not fizz with acid 2

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3 (a) Solutions FA 6 and FA 7 each contain one cation and one anion. All the ions are listed in the Qualitative analysis notes. (i) Carry out the following tests, using a 1 cm depth of FA 6 or FA 7 in a test‑tube for each test. Complete the table below. Table 3.1 observations test FA 6 FA 7 Test 1 Add an equal volume of aqueous potassium iodide, then add excess aqueous sodium thiosulfate. Test 2 Add a small spatula measure of zinc powder. Leave the mixture to stand. Test 3 Add a few drops of aqueous silver nitrate. Test 4 Add aqueous sodium hydroxide. [5] (ii) Construct an ionic equation for one of the reactions taking place in Test 2. Include state symbols. ....................................................................................................................................... [1] (b) FA 8 contains one anion and one cation. One of these ions contains nitrogen. Both ions are listed in the Qualitative analysis notes. (i) Transfer a small spatula measure of FA 8 into a hard‑glass test‑tube. Heat the test-tube gently at the start, then strongly until no further change occurs. Leave the test-tube to cool. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Carry out further tests to identify each ion in FA 8. Record, in a table in the space below, the reagents, conditions and observations for the tests that positively identify each ion. You may wish to use the following page for rough working. Deduce the chemical formula of FA 8. You must use a boiling tube if any liquid is heated. The formula of FA 8 is .............................. . [6] [Total: 14] Use this page for any rough working.

Mark scheme: 3(a)(i) Correct observations and no extra reagents used test FA 6 FA 7 KI ppt AND brown / yellow-brown / orange-brown / red- brown * (colour of ppt need not be given but allow shades of white and the browns given above) red-brown / brown solution OR turns red-brown / brown * then thio brown colour fades / disappears OR white / off-white / cream ppt OR ppt dissolves (in excess thio) * (Insoluble in excess is incorrect) (If the colour of solution is given it must be colourless, pale yellow or pale brown.) colourless solution (forms) OR (solution) turns colourless / decolourised * (The formation of the initial purple colour of the thiosulfate complex or the final white / off-white ppt (on standing) are not part of this mark.) Zn solution becomes paler blue / colourless OR brown / black / pink-brown / red-brown ppt / solid (formed) * fizzing / bubbles / effervescence * (gas) pops with lighted splint OR (gas) burns with a pop* AgNO3 white ppt (formed) * white / off-white / cream / pale yellow ppt (formed) * NaOH (pale) blue ppt AND insoluble in excess * rust / brown / orange-brown / red-brown ppt (which does not dissolve in excess) * 2 * = 1 mark 5 Question Answer Marks 3(a)(ii) Any one of the following ionic equations with correct state symbols:  Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)  Zn(s) + 2H+(aq) → H2(g) + Zn2+(aq)  3Zn(s) + 2Fe3+(aq) → 2Fe2+(aq) + 3Zn2+(aq) 1 3(b)(i) Award any two of the following:  Sublimation occurs OR (white) solid condenses / reforms (on sides of tube) OR white smoke OR white fumes  Liquid / water forms OR frothing OR (liquid) condensation OR steam produced OR solid melts OR solid dissolves  (Moist red) litmus turns blue  Residue is colourless / white / off-white / light grey / cream (on cooling) OR no residue  (Gas / fumes) turn (blue) litmus red (Litmus tests must be mentioned in the right order to award both •) 2 3(b)(ii) M1 ‘table’ (2  2 min) with headings: ‘test / experiment / reagents’ and ‘observations’ (owtte) AND two (or more) reagents listed in the space (Cation test) M2 Warm / heat with NaOH (and without Al) M3 Gas / effervescence / NH3 turns (red) litmus blue (Anion test) M4 Add (aqueous) BaCl2 / Ba(NO3)2 AND to a solution of FA 8 M5 White ppt (forms) AND either ppt is insoluble in (excess) HCl / HNO3 or FA 8 / mixture does not decolorise (acidified) potassium manganate(VII) / KMnO4 M6 FA 8 is (NH4)2SO4 6

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