Cambridge A Level Chemistry 9701 — 2024 May/June Paper 3 · Variant 3

1 When ammonium compounds are heated with sodium hydroxide or calcium hydroxide, ammonia is liberated. The ionic equation for the reaction is shown. NH4+ + OH− NH3 + H2O FA 1 is prepared by heating 10.0 cm3 of aqueous ammonium chloride, NH4Cl (aq), with 25.0 cm3 of 2.00 mol dm−3 sodium hydroxide, NaOH. The sodium hydroxide is in excess. The reaction mixture is cooled and diluted to 250 cm3 with distilled water. You will determine the concentration of the aqueous solution of ammonium chloride by titrating the remaining sodium hydroxide from the preparation of FA 1 with a known concentration of sulfuric acid. 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) FA 2 is 0.0520 mol dm−3 sulfuric acid, H2SO4. FA 3 is bromophenol blue 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 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 in the space below, all your burette readings and the volume of FA 2 added in each accurate titration. Results 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) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sulfuric acid present in the volume of FA 2 in (b). amount of H2SO4 = .............................. mol [1] (iii) Use your answer to (c)(ii) to calculate the amount, in mol, of sodium hydroxide in 25.0 cm3 of FA 1. amount of NaOH = .............................. mol [1] (iv) Use your answer to (c)(iii) and the information given to calculate the amount, in mol, of sodium hydroxide that reacted with the ammonium chloride when FA 1 was prepared. amount of NaOH that reacted with NH4Cl = .............................. mol [2] (v) Use your answer to (c)(iv) and the information given to calculate the concentration, in mol dm−3, of the ammonium chloride solution used to prepare FA 1. concentration of NH4Cl = .......................................... mol dm−3 [1] (d) When ammonia gas is prepared by heating one of its salts with calcium hydroxide, it can be dried by passing it through a drying agent. Drying agents for gases include calcium oxide, calcium sulfate, concentrated sulfuric acid and phosphorus(V) oxide. Select one of these drying agents and suggest why it is not suitable for drying ammonia gas. ...................................................... is not suitable because ...................................................... ............................................................................................................................................. [1] [Total: 15]

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 II Appropriate headings and units shown 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 / FA 2 AND used / added  unit: / cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded Ill All accurate burette readings are recorded to the nearest 0.05 cm3. lV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 7 Accuracy marks Round all burette readings to the nearest 0.05 cm3. Check and correct subtractions. 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 supervisor’s mean titre to 2 decimal places. Calculate the candidate’s mean titre to 2 decimal places. Calculate the difference  between the candidate’s titre and the supervisor’s titre. Award V if  ⩽ 0.50 (cm3) Award VI if  ⩽ 0.30 Award VII if  ⩽ 0.20 If supervisor’s titre is < 10.0 cm3, tolerances are 0.10, 0.15, 0.25 cm3. Question Answer Marks 1(b) Correctly calculates the mean titre to 2 decimal places.  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 shown next to the two (or more) accurate readings selected.  The mean should be quoted to 2 decimal places and be rounded to nearest 0.01 cm3. 1 1(c)(i) answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) are given to 3 or 4 significant figures 1 1(c)(ii) Correctly calculates amount of H2SO4 in mean titre = 0.052  (b)/1000 (mol) 1 1(c)(iii) Correctly uses amount of NaOH left in 25 cm3 = (c)(ii)  2 (mol) 1 1(c)(iv) Correctly uses M1 initial amount of NaOH = 2.00  25.0/1000 = 0.05(00) (mol) OR (c)(iii)  10 (mol) M2 amount of NaOH reacting with NH4Cl = 0.05 – (c)(iii)  10 (mol) 2 1(c)(v) Correctly uses concentration of NH4Cl = (c)(iv)  1000/10 (mol dm−3) 1 1(d) selects either (concentrated) sulfuric acid OR phosphorus(V) oxide AND because an acidic agent would neutralise / react with NH3 1

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2 A sample of ammonium bromide has been accidentally mixed with another salt. A student suggests that the percentage purity of the ammonium bromide can be determined by measuring the change in temperature when a solution is prepared. The enthalpy change when the sample dissolves in water, ΔH, can then be calculated and compared with the literature value for ammonium bromide. You will carry out the student’s experiment. FA 4 is ammonium bromide, NH4Br, mixed with another salt. (a) Method ● Support the cup in the 250 cm3 beaker. ● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of distilled water into the cup. ● Place the thermometer in the water and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of the water in the space for results. ● Weigh the container with FA 4. Record the mass. ● Add all of FA 4 to the water in the cup. ● Stir the mixture gently so that FA 4 dissolves. ● Measure and record the minimum temperature reached. ● Calculate and record the change in temperature. ● Weigh the container with any residual FA 4. Record the mass. ● Calculate and record the mass of FA 4 added to the cup. Results I II III IV [4] (b) (i) Calculate the energy change, in J, for the reaction. energy change = .............................. J [1] (ii) When one mole of ammonium bromide dissolves in water the enthalpy change, ΔH, is +16.8 kJ mol−1. Use this value to calculate the amount, in mol, of ammonium bromide in the 25.0 cm3 of solution FA 4 used in your experiment. Assume that the enthalpy change when the other salt dissolves in water is 0 kJ mol−1. amount of NH4Br in 25.0 cm3 = .............................. mol [1] (iii) Calculate the mass, in g, of ammonium bromide in your sample. (If you were unable to calculate the amount of ammonium bromide in (b)(ii), then assume it is 2.06 × 10−2 mol. This may not be the correct answer.) mass of NH4Br in sample = .............................. g [1] (iv) Hence calculate the percentage by mass of ammonium bromide in your sample. % by mass of NH4Br = .............................. % [1] (c) Another student states that this method is not accurate as the enthalpy change when the other salt dissolves in water could be exothermic. Suggest how the calculated value in (b)(iv) would change if the other salt dissolves in water exothermically. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (d) The maximum uncertainty in a single thermometer reading is ±0.5 ºC. Calculate the maximum percentage error in your temperature change recorded in (a). Show your working. maximum percentage error = .............................. % [1] [Total: 10] 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. If a solid is heated, a hard-glass test-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 (six headings) AND correctly displayed units  (mass of) container + FA 4  (mass of) container (+ residue)  (mass of) FA 4  temperature of water / initial thermometer reading  minimum / lowest thermometer reading / temperature  change in temperature / temp / T or T Units: /g, (g), in g; / °C, (°C), in °C to cover each entry II Balance readings consistent to either 2 or 3 decimal places AND thermometer readings to .0 °C or .5 °C AND initial thermometer reading is > 10.0 °C and < 40.0 °C III Correctly calculates  mass of solid  change in temperature 4 Accuracy marks Correct temperatures to nearest .5 °C. IV Award for  =  1.0 °C 2(b)(i) Correctly calculates energy change = mcT = 25  4.18  T (J) AND answer to 2–4 sf 1 2(b)(ii) Correctly uses amount of NH4Br = (b)(i)/(16.8  1000) (mol) AND answer to 2–4 sf 1 2(b)(iii) Correctly uses mass of NH4Br = (b)(ii)  97.9 g AND answer to 2–4 sf 1 Question Answer Marks 2(b)(iv) Correctly uses {(b)(iii) / mass in (a)}  100 AND answer to 2–4 sf 1 2(c) temperature decrease is smaller AND (apparent) moles NH4Br is smaller AND percentage by mass will be smaller 1 2(d) Correct expression {(2 x 0.5) / T in (a)}  100 AND answer to 2–4 sf 1

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3 (a) FA 5 is a mixture containing two cations and two anions. Three of the ions are listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Table 3.1. Table 3.1 test observations Test 1 Heat a spatula measure of FA 5 in a hard-glass test-tube. Test 2 Put a 2 cm depth of dilute nitric acid in a boiling tube. Add a spatula measure of FA 5. Dilute the solution from Test 2 with an equal volume of distilled water. Keep the resulting solution for use in (a)(ii). This solution is FA 6. [3] (ii) Carry out the following tests using a 1 cm depth of FA 6 in a test-tube for each test. Record your observations in Table 3.2. Table 3.2 test observations Test 1 Add a 1 cm depth of sodium chlorate(I), then add aqueous sodium thiosulfate dropwise. Test 2 Add aqueous sodium hydroxide. Test 3 Add aqueous ammonia. Test 4 Add a 1 cm depth of aqueous copper(II) sulfate, then add aqueous sodium thiosulfate dropwise until no further change occurs. Test 5 Add a few drops of aqueous silver nitrate, then add aqueous ammonia until no further change occurs. [4] (iii) FA 5 reacts with nitric acid to make FA 6 in Test 2 in Table 3.1. State a change you would see in one of the observations in Table 3.2 if hydrochloric acid had been used to prepare FA 6. ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Use your observations in (a)(i) and (a)(ii) to identify the formulae of three of the ions present in FA 5. Formulae of ions present are ......................... and ......................... and ......................... . [2] (b) FA 7, FA 8 and FA 9 are each known to be propan-1-ol, propanal, propanoic acid or propanone. No two liquids are identical. (i) You will carry out tests to identify samples of FA 7, FA 8 and FA 9. Test 1 has been carried out for you. Carry out Test 2 and record your observations in Table 3.3. Table 3.3 observations test FA 7 FA 8 FA 9 Test 1 orange precipitate orange precipitate no change Put a 1 cm depth in a test- tube. Add a few drops of 2,4-dinitrophenylhydrazine (2,4-DNPH reagent). Test 2 Put a 1 cm depth in a test- tube. Add a small spatula measure of solid sodium carbonate. [1] (ii) State what you can deduce from Tests 1 and 2 in Table 3.3 about the functional group present in each of FA 7, FA 8 and FA 9. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 3(a)(i) Test 1  condensation / droplets on wall (of test-tube) / moisture  purple gas / purple vapour  yellow solid (forms on heating) OR turns yellow  (yellow residue / solid) pales on cooling OR yellow turns white / cream / off-white Test 2  fizz / bubbling / effervescence  gas / CO2 tested with limewater  white ppt formed (with limewater)  colourless / (pale) yellow solution formed Two points needed for each mark. [3] 3(a)(ii) Test 1 + NaClO(aq): • brown/ red-brown / orange- brown / yellow-brown / yellow (solution formed) + Na2S2O3(aq): • colourless solution Test 2 + NaOH(aq): • white ppt AND soluble in excess Test 3 + NH3(aq): • white ppt AND soluble in excess Test 4 + CuSO4(aq): • (white / off-white / cream / brown / yellow-brown) ppt formed OR (any intensity of) brown / yellow-brown (soln) + Na2S2O3(aq): • (then) changes to white ppt OR blue solution Test 5 + AgNO3(aq): • (pale) yellow ppt + NH3(aq): • ppt insoluble OR ppt turns paler Two points needed for each mark. 4 Question Answer Marks 3(a)(iii) white ppt (as well as yellow) AND in Test 5/with AgNO3 1 3(a)(iv) ions present: I, Zn2+, CO32 2 correct = 1 mark; 3 correct = 2 marks 2 3(b)(i) FA 7 and FA 8 no reaction / no change / no bubbling AND FA 9 fizzing / effervescence / bubbling 1 3(b)(ii) M1 FA 7 and FA 8 are carbonyl compounds / are either aldehydes OR ketones / either propanal OR propanone M2 FA 9 contains carboxyl (group) / is carboxylic acid / propanoic acid (from correct result with Na2CO3) 2 3(b)(iii) M1 add (acidified aqueous) potassium manganate(VII)/ KMnO4 M2 purple to colourless / (pale) brown / KMnO4 decolourises and with FA 8 AND FA 7 is propanone, FA 8 is propanal 2

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