Cambridge A Level Chemistry 9701 — 2022 May/June Paper 3 · Variant 1

1 In this experiment you will identify a straight-chain carboxylic acid by titrating an aqueous solution of this acid with aqueous sodium hydroxide. 1 mole of the carboxylic acid reacts with 1 mole of sodium hydroxide. The carboxylic acid contains C, H and O atoms only and has no C=C bonds. FA 1 is an aqueous solution of the carboxylic acid, containing 10.50 g dm–3. FA 2 is 0.110 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 approximately 8 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 obtain 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 your answer to (c)(i) and the information on page 2 to calculate the relative formula mass of the carboxylic acid in FA 1. Mr of carboxylic acid = .............................. [1] (iii) Identify the carboxylic acid in FA 1. Draw its skeletal formula. skeletal formula name of acid ........................................................................................................................ [2] (d) A student carries out a similar titration to the titration you carried out in (a). The only difference is that a solution of aminoethanoic acid, NH2CH2CO2H, containing 10.50 g dm–3 is used instead of the acid in FA 1. (i) Construct an equation for the reaction taking place in the student’s titration. Include state symbols. ....................................................................................................................................... [1] (ii) State whether the student’s titre will be larger or smaller than your titre. Explain your answer. The student’s titre will be .............................. than mine. explanation .......................................................................................................................... ............................................................................................................................................. ....................................................................................................................................... [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 1 II Appropriate headings and units in the accurate titration table and titre values recorded for accurate titrations  initial / start and (burette) reading / volume  final / end and (burette) reading / volume  titre or volume used / added / or FA 2 added  unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded 1 III All accurate burette readings are recorded to the nearest 0.05 cm3 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 For assessment of accuracy marks, round all burette readings to the nearest 0.05 cm3. Check and correct subtractions where necessary. 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 3 Question Answer Marks 1(b) Candidate must average two (or more) titres that are within a total spread of not more than 0.20 cm3 AND working / explanation must be shown or ticks must be put next to the two (or more) accurate titres selected AND mean quoted to 2 decimal places 1 1(c)(i) Correctly calculates moles of NaOH used = 0.110  (b) / 1000 AND answer to 3 or 4 sig fig 1 1(c)(ii) Correctly uses (c)(i) to calculate Mr = 10.5 / (c)(i)  40 1 1(c)(iii) M1 Identity of carboxylic acid [must be consistent with the Mr in (c)(ii)] M2 Skeletal formula (must correspond to candidate’s name of acid) 2 1(d)(i) Correct equation with state symbols NH2CH2COOH(aq) + NaOH(aq)  NH2CH2COONa(aq) + H2O(l) 1 1(d)(ii) Student’s titre would be larger AND Mr of amino acid is 75 / is lower than Mr of FA 1 so more moles of amino acid are present ORA 1

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2 In this experiment you will identify a magnesium compound by thermal decomposition. When heated this compound decomposes to give magnesium oxide. FA 4 is the magnesium compound. (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 the mass of FA 4. Record the mass. ● Place the crucible and contents on a pipe-clay triangle. ● Heat the crucible gently, without the lid, for approximately 2 minutes. ● Heat strongly for a further 4 minutes. ● Place the lid on the crucible and leave it 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, without the lid, 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 the mass of residue obtained. Record the mass. Results I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of magnesium oxide produced in your experiment. amount of MgO = .............................. mol [1] (ii) 1 mole of FA 4 decomposes on heating to produce 1 mole of MgO and 1 mole of gas X. Calculate the relative formula mass, Mr , of X. Mr of X = .............................. [1] (iii) X contains one or more oxygen atoms. Suggest the identity of X. X is .............................. . [1] (iv) Deduce the name of FA 4. FA 4 is ........................................................................ . [1] (c) A student suggests that this experiment will be more accurate if FA 4 is heated throughout the experiment with a lid on the crucible. State whether the student is correct. Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (d) State the uncertainty in a single reading of your balance. uncertainty = ± .............................. g Calculate the maximum percentage error in the mass of residue that you obtained. Show your working. maximum percentage error = ..............................% [1] [Total: 11] 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:  (mass of) crucible, lid (empty)  (mass of) crucible, lid and FA 4 (or ‘contents before heating’)  (mass of) crucible, lid and MgO / residue after first heating  (mass of) crucible, lid and MgO / residue after second heating 1 II Readings are appropriately recorded:  all weighings recorded to same decimal places (two or more)  mass of FA 4 is within range 0.80 g–1.80 g (from weighings)  fourth weighing within +0.02 and –0.05 g of third weighing 1 III Correct subtractions to give masses of FA 4 and MgO / residue 1 IV and V: Accuracy marks Calculate supervisor’s mass ratio (to 2 d.p.) = mass FA 4 / mass of residue Calculate the candidate’s mass ratio (to 2 d.p.) = mass FA 4 / mass of residue Calculate 20% of this ratio and calculate ratio  this ratio Award IV if  is within the range 0.00–0.25 Award V if  is within the range 0.00–0.10 2 2(b)(i) Correctly calculates moles of MgO = mass of residue / 40.3 AND answer to 2–4 sig fig 1 2(b)(ii) Correctly uses (b)(i) to calculate Mr of X = mass lost / moles of MgO OR Mr of X = (mass FA 4 / moles of MgO) – 40.3 AND answer to 2–4 sf 1 2(b)(iii) X is water / steam / H2O / CO2 1 2(b)(iv) FA 4 is magnesium hydroxide 1 Question Answer Marks 2(c) Student is not correct because there is no spitting / frothing during heating OR student is correct because there was spitting / frothing during heating 1 2(d) 2 d.p. balance uncertainty = 0.01 g or 0.005 g 3 d.p. balance uncertainty = 0.001 g or 0.0005 g Correct expression for % error = (2  balance uncertainty / mass of residue)  100 1

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3 (a) FA 5 is an ionic solid containing two ions. It contains one or more ions that contain nitrogen. (i) Carry out suitable tests to identify the anion. Reserve a small amount of FA 5 for use in (a)(ii). Record the tests you carry out and the observations you make, in a table, in the space below. You must use a boiling tube if any liquid is heated. anion in FA 5 = .............................. [4] (ii) Heat a small spatula measure of FA 5 in a hard-glass test-tube. When no further change occurs, allow the tube and its contents to cool completely. Record all the observations you make and any subsequent conclusions. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (b) FA 6 is a solution of a compound containing one cation and one anion, both of which are in the Qualitative analysis notes. FA 7 is an aqueous mixture of two substances. FA 7 contains one potassium-containing compound and one other substance. All substances are listed in the Qualitative analysis notes. (i) Carry out the following tests. Complete the table below. Use a 1 cm depth of FA 6 or FA 7 in a test-tube for each test. Table 3.1 observations test FA 6 FA 7 Test 1 Add aqueous sodium hydroxide. Test 2 Add aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid. Test 3 Add a few drops of aqueous starch, then add aqueous sodium thiosulfate. Test 4 Add a few drops of aqueous silver nitrate, then add a few drops of aqueous sodium hydroxide. Test 5 Add aqueous ammonia. [5] (ii) Give the formulae of the substances in FA 6 and FA 7. FA 6 is .................................................... . FA 7 contains ................................................... and ��������������������������������������������������� . [3] (iii) Give the ionic equation for one of the reactions taking place in Test 1. Include state symbols. ....................................................................................................................................... [1] [Total: 15]

Mark scheme: 3(a)(i) AND two (or more) reagents listed in the space M2: (eliminating ammonium ion) heat FA 5 with (aqueous) NaOH AND no effervescence / (damp red) litmus stays red M3: (identifying NO2– / NO3–) add Al to warm NaOH and FA 5 AND fizz / gas / NH3 turns (damp red) litmus blue M4: (eliminating NO2−) anion is NO3− / nitrate / nitrate(V) AND Either add (a few drops of) (acidified) KMnO4 / potassium manganate(VII) AND no change / no reaction / (solution) remains purple Or add (dilute) named mineral acid to (solid or aqueous) FA 5 AND no brown fumes / no blue solution / no reaction / no change produced 4 Question Answer Marks 3(a)(ii)  solid melts / solid dissolves / liquid forms  fizzing / bubbling / effervescence  (gas) re-lights glowing splint / spill  oxygen produced  (on cooling), (pale) yellow solid formed or residue is yellow 3 or more bullets = 2 marks 2 bullets = 1 mark 2 Question Answer Marks 3(b)(i) test FA 6 FA 7 Test 1 NaOH White ppt / solid (formed) * Soluble in excess * Decolourises / turns (pale) yellow * (ppt is CON) Test 2 Ba2+ White ppt No change / no reaction + HCl AND ppt insoluble / remains / no change / no reaction * AND no change / no reaction * Test 3 starch Dark blue / blue-black / black (colour formed) (ignore state) + thio AND colourless solution (forms) ALLOW turns colourless / decolourises * Test 4 Ag+ No change / no reaction / no ppt Yellow / brown ppt (forms) * + NaOH AND ppt (forms) (ignore colour) * Pale yellow ppt ALLOW ppt turns paler yellow * IGNORE use of excess NaOH Test 5 + NH3 White ppt AND ppt is insoluble in excess (NH3) * 2 * = 1 mark (round down) 5 3(b)(ii) FA 6 = Al 2(SO4)3 [1] FA 7 = I2 [1] AND KI [1] 3 Question Answer Marks 3(b)(iii) Al 3+(aq) + 3OH–(aq)  Al (OH)3(s) OR Al (OH)3(s) + OH−(aq)  [Al (OH)4]−(aq) OR I2(aq) + 6OH–(aq)  I–(aq) + IO3–(aq) + 3H2O(l) 1

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