Cambridge A Level Chemistry 9701 — 2023 May/June Paper 3 · Variant 2

1 In this experiment you will determine the relative atomic mass, Ar , of metal M by thermal decomposition of its basic carbonate, MCO3•M(OH)2. FB 1 is the basic metal carbonate, MCO3•M(OH)2. (a) Method • Weigh the empty crucible with its lid. Record the mass. • Transfer all of the FB 1 from the container into the crucible. • Weigh the crucible, lid and FB 1. Record the mass. • Calculate and record the mass of FB 1 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 is cool, weigh the crucible with its lid and contents. Record the mass. • Place the crucible and contents on the pipe-clay triangle. Remove the lid. • Heat the crucible strongly for a further 2 minutes. • Replace the lid and leave the crucible to cool for at least 5 minutes. • When the crucible is cool, reweigh the crucible with its lid and contents. Record the mass. • Calculate and record the mass of residue obtained. Results I II III IV V [5] (b) Calculations (i) When FB 1 undergoes thermal decomposition, the products are the metal oxide, MO, carbon dioxide and water vapour. Give the equation for the thermal decomposition of FB 1. Include state symbols. ..................................................................................................................................... [1] (ii) The amount, in mol, of carbon dioxide produced is given by the following formula. mass loss during heating amount of CO2 = (Mr of CO2 + Mr of water) Calculate the amount, in mol, of carbon dioxide produced in (a). amount of CO2 = ................................ mol [1] (iii) Calculate the relative formula mass, Mr , of the basic metal carbonate. Show your working. Mr of MCO3•M(OH)2 = ....................................... [1] (iv) Calculate the relative atomic mass of metal M. Ar of M = ....................................... [1] (c) A student accidentally spilt a little of the residue before carrying out the final weighing. Predict whether the calculated value of the relative atomic mass of M will be higher or lower as a result of this mistake. Explain your answer. The Ar of M will be ............................. . explanation ............................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [1] (d) A student suggested that addition of sulfuric acid to the residue from (a) would show whether the basic metal carbonate had decomposed fully. State whether the student is correct. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 11]

Mark scheme: 1(a) I Unambiguous headings and units for four weighings  (Mass of) crucible, lid (empty)  (Mass of) crucible, lid and FB 1 (or “contents before heating”)  (Mass of) crucible, lid and residue / MO / contents after first heating  (Mass of) crucible, lid and residue / MO / contents after 2nd heating  Units must be ‘ / ’ or ‘in’ or “()” AND g or ‘grams’ in column headings or with every entry II Four weighings are recorded in space provided  All weighings recorded to same decimal places (2 or 3)  Fourth weighing is within +0.02 and -0.04 g of third weighing III Correct subtractions to give masses of FB 1 and MO / residue.  These values (with unambiguous headings) must be on page 2 Accuracy marks Calculate supervisor’s mass ratio (to 2 d.p.) = mass FB 1 / mass of residue Calculate candidate’s mass ratio (to 2 dp) = mass FB 1 / mass of residue Difference () between the candidate’s mass ratio and the supervisor’s mass ratio IV if  is within 25% of the supervisor ratio V if  is within 10% of the supervisor ratio 1(b)(i) MCO3•M(OH)2(s)  2MO (s) + CO2(g) + H2O (g) 1 1(b)(ii) Correctly calculates amount of CO2 produced n(CO2) = mass lost / (44 + 18) AND answer to 2–4 significant figures 1 1(b)(iii) Correct use of (b)(ii) to calculate Mr Mr = mass of FB 1 used / mol of CO2 produced AND answer to 2–4 significant figures 1 1(b)(iv) Correct use of Mr to calculate Ar Ar = [Mr – (60  34)] / 2 AND answer to 2–4 significant figures 1 Question Answer Marks 1(c) relative atomic mass will be lower AND mass loss will be greater OR amount (in mol) of CO2 (and water) will be greater AND amount (in mol) of basic carbonate (calculated) will be greater OR Mr lower 1 1(d) student is not correct AND sulfuric acid will not show if the metal hydroxide is still present OR student is correct AND if decomposition was not complete, residue would fizz with acid OR if decomposition was complete, residue would not fizz with acid OR carries out experiment and gives correct conclusion based on observation 1

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2 In this experiment you will determine the relative atomic mass, Ar, of another metal, X, by a titration method using the metal carbonate, X2CO3. FB 2 is 0.0460 mol dm–3 hydrochloric acid, HCl. FB 3 is the metal carbonate, X2CO3. FB 4 is methyl orange indicator. (a) Method Preparing a solution of FB 3 • Weigh the stoppered container of FB 3. Record the mass in the space below. • Tip all of the FB 3 into the 250 cm3 beaker. • Reweigh the container with its stopper. Record the mass. • Calculate and record the mass of FB 3 used. • Add approximately 100 cm3 of distilled water to FB 3 in the beaker. • Stir the mixture with a glass rod until all the FB 3 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 X2CO3 is FB 5. Label the flask FB 5. Titration • Fill the burette with FB 2. • Pipette 25.0 cm3 of FB 5 into a conical flask. • Add several drops of FB 4 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 FB 2 added in each accurate titration. I II III IV V VI VII VIII [8] (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 FB 5 required ........................... cm3 of FB 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iv), (c)(v) and (c)(vi) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of hydrochloric acid present in the volume of FB 2 in (b). amount of HCl = ................................ mol [1] (iii) Give the ionic equation for the reaction of hydrochloric acid with the metal carbonate during the titration. Include state symbols. .....CO32–..... + ............................ ............................ + ............................ [1] (iv) Calculate the concentration of X2CO3, in mol dm–3, in FB 5. concentration of X2CO3 in FB 5 = ................................ mol dm–3 [1] (v) Calculate the relative formula mass, Mr , of X2CO3. Mr of X2CO3 = ....................................... [1] (vi) Calculate the relative atomic mass of X. Ar of X = ....................................... [1] (vii) Identify X. X is ..................................... . [1] [Total: 16] 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 Data from weighings of FB 3 recorded in space provided  three masses recorded, with unambiguous headings and unit  subtraction correct (for mass of FB 3)  mass of FB 3 used is between 0.80 and 1.80 g (from weighings) 8 II 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 III 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 / FB 2 AND used / added  unit: / cm3 or (cm3) or “in cm3” (for each heading) or cm3 unit given for each volume recorded IV All accurate burette readings are recorded to the nearest 0.05 cm3 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre Accuracy marks The Examiner should 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. Calculate the Supervisor’s ratio (mean titre / mass of FB 3) to two decimal places. Calculate the candidate’s ratio (mean titre / mass of FB 3) to two decimal places. Calculate the difference () between the candidate’s ratio and the supervisor’s ratio. Award VI if  ⩽ 0.80 Award VII if  ⩽ 0.50 Award VIII if ⩽ 0.30 (cm3 g-1) Question Answer Marks 2(b) Correctly calculates the mean titre  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 dp and be rounded to nearest 0.01 cm3 (e.g. 26.625 cm3 must be rounded to 26.63 cm3) 1 2(c)(i) All answers to parts (c)(ii), (c)(iv), (c)(v) and (c)(vi) given to 3 or 4 significant figures. 1 2(c)(ii) Correct calculation of number of moles of HCl number of moles = vol in (b) x 0.046 / 1000 1 2(c)(iii) Ionic equation correct (including state symbols) CO32– (aq) + 2H+ (aq)  CO2(aq) + H2O(l) 1 2(c)(iv) Correct use of (b), (c)(ii) and (c)(iii) to calculate concentration of X2CO3 [X2CO3] = 0.5  (c)(ii)  1000 / 25mol dm–3 1 2(c)(v) Calculation of Mr of FB 3 Mr of X2CO3 = 4  mass FB 3 used / (c)(iv) 1 2(c)(vi) Calculation of Ar from Mr Ar of X = (Mr – 60) / 2 1 2(c)(vii) Appropriate name / symbol of X X must be in Group 1 and have nearest Ar to value calculated in (c)(vi) 1

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3 (a) FB 6 is a solution containing one cation listed in the Qualitative analysis notes. The anion contains sulfur. (i) State the reagents you would use to identify the cation in FB 6. reagents ............................................................................................................................ Use your selected reagents to test FB 6. Use 1 cm depth of FB 6 in a test-tube for each test. Record your observations in the space below. [2] (ii) Identify the anion in FB 6. Include a description of your procedure and the observations you make. anion in FB 6 ....................................... [2] (iii) Deduce the formula of FB 6. formula of FB 6 ....................................... [1] (b) You will devise chemical tests to distinguish between the two possible identities given for each of compounds FB 7, FB 8, FB 9 and FB 10. In each case, you should: • name the reagent or reagents you will use to identify the compound • state any necessary conditions for your test • use a 1 cm depth of the solution of the unknown compound and use a boiling tube if you need to warm a mixture • carry out your test and record the observations you make (if any) • state your conclusion about the identity of the compound. (i) FB 7 is either aqueous sodium nitrate or aqueous sodium nitrite. FB 7 is ..................................... . [2] (ii) FB 8 is either aqueous sodium nitrate or aqueous silver nitrate. FB 8 is ..................................... . [2] (iii) FB 9 is either aqueous ethanol or aqueous propan-1-ol. (In your test, do not heat but you may need to leave your reaction mixture to stand.) FB 9 is ..................................... . [2] (iv) FB 10 is either aqueous methanol or aqueous ethanoic acid. FB 10 is ..................................... . [2] [Total: 13]

Mark scheme: 3(a)(i) No mark for reagents (NaOH and NH3) M1 NaOH: white ppt AND insoluble in excess (NaOH) M2 NH3: white ppt AND insoluble in excess (ammonia) 3(a)(ii) M1 Add (aq) BaCl 2 or Ba(NO3)2 AND gives a white ppt M2 ppt is insoluble in excess (dil) HCl or (dil) HNO3 (not H2SO4) OR FB 6 does not decolourise KMnO4 / KMnO4 remains purple 2 3(a)(iii) MgSO4 1 3(b) For each part award one mark for suitable reagent(s) to distinguish between the suggested identities. Then award one mark for correct observation and conclusion. 3(b)(i) FB 7 M1 Add (a few drops of) (acidified) KMnO4 M2 KMnO4 goes colourless / decolorised AND FB 7 is (sodium) nitrite / NaNO2 2 3(b)(ii) FB 8 M1 M2 Add dilute HCl or BaCl2  white ppt OR add aq Na2CO3  white / beige / pale-brown / off white / cream ppt OR add (aq) NaOH  brown ppt OR add Mg  black ppt AND FB 8 is silver nitrate / AgNO3 2 3(b)(iii) FB 9 Add (aqueous) iodine AND (aqueous) NaOH Gives (pale) yellow / cream / off-white precipitate AND FB 9 is ethanol / CH3CH2OH / C2H5OH 2 Question Answer Marks 3(b)(iv) FB 10 Add Mg (strip) OR Na2CO3 OR named indicator OR warm with (a few drops of) (acidified) KMnO4 Fizzing OR correct final colour of indicator OR KMnO4 stays purple (or no change) AND FB 10 is ethanoic acid / CH3COOH / CH3CO2H 2

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