Cambridge A Level Chemistry 9701 — 2025 May/June Paper 3 · Variant 5

1 Group 1 metal carbonates react with dilute acids to form a salt, water and carbon dioxide. The identity of the metal, M, can be determined by measuring the volume of carbon dioxide produced when excess acid is added to a known mass of the carbonate, M2CO3. FA 1 is the metal carbonate, M2CO3. FA 2 is 0.250 mol dm−3 sulfuric acid, H2SO4. (a) Method • Weigh the container with FA 1. Record the mass in the space for results. • Fill the tub with water to a depth of approximately 5 cm. • Fill the 250 cm3 measuring cylinder completely with water. Holding a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. • Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub. • Use the 50 cm3 measuring cylinder to transfer 50.0 cm3 of FA 2 into the flask labelled X. Check that the bung fits tightly into the neck of flask X, clamp flask X and place the delivery tube into the inverted 250 cm3 measuring cylinder. • Remove the bung from the neck of the flask. Add all of the FA 1 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. • Replace the flask in the clamp and leave until the fizzing stops. Swirl the flask occasionally. • Weigh the container with any residual FA 1. Record the mass. • Calculate and record the mass of FA 1 that is added to the acid. • When no more gas is collected, record the final volume of gas. You may wish to start Question 2 or Question 3 while the gas is being collected. Results I II III [3] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide collected in the measuring cylinder (at room conditions). amount of CO2 = .............................. mol [1] (ii) Write an equation for the reaction between sulfuric acid and the metal carbonate, M2CO3. Include state symbols. ..................................................................................................................................... [1] (iii) Use your answers to (b)(i) and (b)(ii) to deduce the amount, in mol, of the metal carbonate in the mass of FA 1 you used in your experiment. amount of M2CO3 = .............................. mol Hence, calculate the relative formula mass, Mr, of the metal carbonate. Mr of M2CO3 = .............................. [1] (iv) Use your answer to (b)(iii) to calculate the relative atomic mass, Ar, of metal M. Hence, identify metal M in FA 1. Ar of M = ................................ M is .............................. . [2] (c) A student carries out a similar experiment on a powdered Group 2 carbonate. The student determines the relative atomic mass, Ar, to be 49.3 and concludes that the metal in the carbonate is calcium. Calculate the percentage error in the student’s result. Show your working. percentage error in Ar of Ca = .............................. % [1] (d) Suggest why the method you used in (a) would be unsuitable for use with small lumps of calcium carbonate. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 10]

Mark scheme: Question Answer Marks 1(a) M1 Unambiguous headings and correct units for 3 • (mass of) container with FA 1 • (mass of) container (+ residual FA 1) • (mass of) FA 1 (added) • (final) volume (of gas produced) Units: / g, (g), in g or g with each mass entry AND / cm3, (cm3), in cm3 or cm3 against the volume entry M2 Both balance readings given consistently to either 2 or to 3 dp AND mass of FA 1 correctly calculated AND volume recorded as integer (cylinder graduations at 2 cm3) M3 125 cm3 ⩽ volume of gas collected ⩽ 250 cm3 1(b)(i) Correctly calculates 1 amount of CO2 = vol in (a) / 24 000 mol AND answer given to 2 – 4 sf 1(b)(ii) M2CO3(s) + H2SO4(aq) → M2SO4(aq) + H2O(l) + CO2(g) 1 1(b)(iii) Correctly uses 1 amount of M2CO3 = (b)(i) mol AND Mr of M2CO3 = mass of FA 1 / (b)(i) AND answer given to 2 – 4 sf 1(b)(iv) M1 Correct working shown 2 Ar of M = ((b)(iii) − 60)/2 M2 Identifies closest group 1 metal Li ⩽ 14.95 ⩽ Na ⩽ 31.05 ⩽ K ⩽ 62.30 ⩽ Rb ⩽ 109.20 ⩽ Cs ⩽ 250. 1(b)(c) percentage error = (49.3 – 40.1)  100 / 40.1 = 22.9 % 1 1(d) The calcium sulfate formed is sparingly / partially soluble in water (so inhibits further reaction) 1 OR a sparingly/partially soluble surface layer is formed.

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2 The identity of a Group 1 metal in a metal carbonate may also be determined by titration with acid. Z2CO3 is a Group 1 metal carbonate. The metal may or may not be the same as that in FA 1. You will determine which metal is present in Z2CO3. FA 3 is 7.26 g dm−3 Z2CO3. FA 4 is 0.0500 mol dm−3 sulfuric acid, H2SO4. FA 5 is bromophenol blue indicator. (a) Method • Fill the burette with FA 4. • Pipette 25.0 cm3 of FA 3 into a conical flask. • Add a few drops of FA 5 to the same 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 4 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 3 required .............................. cm3 of FA 4. [1] (c) Calculations (i) Calculate the amount, in mol, of sulfuric acid present in the volume of FA 4 in (b). amount of H2SO4 = .............................. mol [1] (ii) Use your answer to (c)(i) to calculate the amount, in mol, of Z2CO3 in 1.0 dm3 of FA 3. amount of Z2CO3 = .............................. mol [1] (iii) Use your answer to (c)(ii) to determine the identity of metal Z. Metal Z is .............................. . [2] (d) Z2CO3 also exists as the hydrated salt. State whether your titre will increase or decrease if 7.26 g of hydrated Z2CO3 is dissolved to prepare 1 dm3 of FA 3. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (e) A titration is a more accurate method of determining the relative atomic mass of the metal in a metal carbonate than gas collection. Give two reasons why titration is a more accurate method. 1 ................................................................................................................................................ ................................................................................................................................................... 2 ................................................................................................................................................ ................................................................................................................................................... [2] [Total: 15] 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 any 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 All the following data recorded 1 • two burette readings and titre for the rough titration • initial AND final burette readings for two (or more) accurate titrations II Titre values recorded for accurate titrations and correct headings and units in the accurate titration table 1 • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre OR volume / FA 4 AND used / added • unit: /cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded III All accurate burette readings are to nearest 0.05 cm3. 1 IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre. 1 Accuracy (Q) marks Round burette readings to the nearest 0.05cm3. Check and correct titre subtractions where necessary. Select the best mean titre, using the following 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. Calculate the candidate’s mean value. Calculate the difference () between the candidate’s mean titre and the supervisor’s mean titre. Award V if  ⩽ 0.50 (cm3) 3 Award VI if  ⩽ 0.30 Award VII if  ⩽ 0.20 2(b) Correct calculation of the mean titre 1 • 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 d.p. and be rounded to nearest 0.01 cm3. 2(c)(i) Correctly calculates 1 amount of H2SO4 = 0.0500  (b) / 1000 mol AND answer given to 3 or 4 sf 2(c)(ii) Correctly uses 1 amount of Z2CO3 = (c)(i)  1000 / 25 mol AND answer given to 3 or 4 sf 2(c)(iii) Correctly uses 2 M1 Mr of Z2CO3 = 7.26/(c)(ii) M2 Ar of Z = (answer− 60)/2 AND correctly identifies Z Li ⩽ 14.95 ⩽ Na ⩽ 31.05 ⩽ K ⩽ 62.30 ⩽ Rb ⩽ 109.20 ⩽ Cs ⩽ 250. 2(d) amount / moles of metal carbonate is less with hydrated salt 1 AND (fewer moles of acid are required so) titre is smaller 2(e) two of the following: 2 • some CO2 / gas dissolved in water (so volume inaccurate) • some CO2 / gas escaped before bung was inserted • concordant titres

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3 Half‑fill the 250 cm3 beaker with water and place it on a tripod and gauze. Heat the water until boiling then switch off your Bunsen burner. This is your hot water bath for use in (b). (a) FA 6 and FA 7 are aqueous solutions that each contain one cation and one anion. All the ions are listed in the Qualitative analysis notes. None of the ions contains nitrogen. (i) Carry out the following tests using a 1 cm depth of FA 6 or FA 7 in a test‑tube for each test. Record your observations in Table 3.1. Table 3.1 observations test FA 6 FA 7 Test 1 Add aqueous sodium hydroxide. Test 2 Add aqueous ammonia, then let the mixture stand for 5 minutes. Test 3 Add hydrogen peroxide, mix well, then add aqueous sodium hydroxide. [4] (ii) Carry out tests to identify the anions in FA 6 and FA 7. Record your tests and observations in a suitable form in the space below. [4] (iii) Use your observations from (a)(i) and (a)(ii) to deduce the formulae of FA 6 and FA 7. FA 6 is .............................. . FA 7 is .............................. . [2] Ensure your water bath is hot and the Bunsen burner is turned off before you start (b). (b) FA 8 is an organic liquid containing one functional group and only the elements C, H and O. You will carry out two tests to investigate FA 8. For each test you will record your observations and then conclude one of the following: • at least two types of compound that FA 8 could be or • one type of compound that FA 8 cannot be. (i) To a 1 cm depth of FA 8 in a test‑tube, add a few drops of acidified aqueous potassium manganate(VII), then place the test‑tube in the hot water bath. observation ........................................................................................................................ ........................................................................................................................................... conclusion ......................................................................................................................... [2] (ii) To a 1 cm depth of FA 8 in a test‑tube, add a small spatula measure of sodium carbonate. observation ........................................................................................................................ ........................................................................................................................................... conclusion ......................................................................................................................... [2] (iii) Suggest a further test you could carry out to identify one of the types of compound that you have concluded could be FA 8. State the reagent you would use. State what a positive result would indicate about the identity of FA 8. Do not carry out your test. reagent .............................................................................................................................. conclusion from a positive result ....................................................................................... ........................................................................................................................................... [1] [Total: 15]

Mark scheme: 3(a)(i) Marks for observations: 2 * = 1 mark 4 Test 1 FA 6 green ppt * insoluble in excess OR ppt turns brown on standing / in air * FA 7 grey-green ppt * soluble in excess OR forms dark green solution * Test 2 FA 7 grey-green ppt insoluble in excess * (pale) purple / mauve / lilac solution on standing * Test 3 FA 6 With peroxide: (solution) turns yellow AND with hydroxide: (dark) brown ppt/solid with NaOH(aq)* In either box (or both) bubbling / effervescence / fizzing * gas relights glowing splint * 3(a)(ii) M1 Tests and observations shown clearly in a table with at least one reagent and one observation listed. Headings must be 4 given. M2 Tests both FA 6 AND FA 7 with one of (aqueous) silver nitrate / barium chloride / barium nitrate (1st reagent) OR Tests FA 6 or FA 7 with two of the reagents given above. M3 FA 6 + BaCl 2(aq) or Ba(NO3)2(aq): white ppt AND insoluble in (dilute) HNO3 or HCl M4 FA 7 + AgNO3(aq): white ppt 3(a)(iii) M1 FA 6 is FeSO4 2 M2 FA 7 is CrCl3 3(b)(i) M1 Observation: KMnO4 / purple (solution) turns colourless / pale yellow / pale brown 2 M2 Conclusion: FA 8 could be any two from 1º alcohol / 2 º alcohol / aldehyde OR FA 8 cannot be ketone / 3º alcohol 3(ii) M1 Observation: no change / no reaction / no effervescence 2 M2 Conclusion: FA 8 is not a (carboxylic) acid 3(iii) One of: 1 • 2,4-DNPH / Brady's reagent AND aldehyde • Tollens’ / Fehling’s / Sandell’s (reagent) AND aldehyde • Na / PCl5 / SOCl2 AND alcohol

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