Cambridge A Level Chemistry 9701 — 2024 May/June Paper 3 · Variant 4

1 In this experiment you will determine the relative formula mass, Mr , of a basic metal carbonate, MCO3•M(OH)2, by a titration method. FB 1 is the basic metal carbonate MCO3•M(OH)2. FB 2 is a solution containing hydrochloric acid, HCl , and MCl 2, prepared using FB 1 as follows. ● 22.50 g of FB 1, MCO3•M(OH)2, is weighed out. ● 100.0 cm3 of 5.00 mol dm–3 hydrochloric acid (a small excess) is added to FB 1. ● The mixture is left to allow FB 1 to react completely. MCO3•M(OH)2(s) + 4HCl (aq) 2MCl 2(aq) + CO2(g) + 3H2O(l) ● The resulting solution is made up to 1.00 dm3 with distilled water. ● This solution is FB 2. FB 3 is potassium hydroxide, KOH, of concentration 5.05 g dm–3. FB 4 is thymolphthalein indicator. (a) Method ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Add a few 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 in the space below, all your burette readings and the volume of FB 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 FB 3 required .............................. cm3 of FB 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of potassium hydroxide present in 25.0 cm3 of FB 3. amount of KOH = .............................. mol [1] (iii) Give the ionic equation for the reaction of hydrochloric acid with potassium hydroxide during the titration. Include state symbols. ........................................................................................................................................... Hence calculate the concentration, in mol dm–3, of hydrochloric acid in FB 2. concentration of HCl = .............................. mol dm–3 [2] (iv) Use the information about FB 2 and your answer to (c)(iii) to calculate the relative formula mass, Mr , of MCO3•M(OH)2. Mr of MCO3•M(OH)2 = .............................. [2] (d) A student suggested that the procedure used in (a) would be more accurate if the mass of FB 1 used to prepare solution FB 2 is doubled. No other change to the procedure is made. Explain why the student is not correct. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 15]

Mark scheme: 1(a) I 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 Correct 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 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. 7 Accuracy marks Round all burette readings to the nearest 0.05cm3 then 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 The ‘best’ titres should be used to calculate the mean titre, expressed to the 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. V Award if  ⩽ 0.50 cm3 VI Award if  ⩽.0.30 cm3 VII Award if  ⩽ 0.20 cm3 If Supervisor’s mean titre < 10.00 cm3 then halve the tolerances (0.25, 0.15, 0.10 cm3). If Supervisor’s mean titre < 5.00 cm3 then use the tolerances 0.15, 0.10, 0.05 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 put 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) and (c)(iv) are given to 3 or 4 significant figures 1 1(c)(ii) Correctly calculates amount of KOH used = 5.05 / 56.1  25 / 1000 mol = 0.00225(0) (mol) OR 2.25(0) x 10−3 (mol) 1 1(c)(iii) M1 ionic equation: H+(aq) + OH-(aq) → H2O(l) M2 correct use of (b) AND (c)(ii) [FB 2 / HCl ] = (c)(ii)  1000 / (b) (mol dm−3) 2 1(c)(iv) Correct use of (c)(iii) M1 mol of HCl reacting with FB 1 = 0.10  5.00 – (c)(iii) M2 amount of FB 1 used = (mol HCl from M1) / 4 AND Mr of FB 1 = 22.5 / amount FB 1 used 2 1(d) the acid / HCl (added to FB 1) would no longer be in excess / HCl becomes the limiting reagent OR no HCl would be left (to do the titration) / insufficient HCl to react with all the FB 1 / (metal) carbonate OR FB 1 / (metal) carbonate is now / will be in excess (so titration will not be possible) 1

More questions on Reacting masses and volumes (of solutions and gases)

2 In this experiment you will determine the relative atomic mass, Ar , of metal M by thermal decomposition of the same basic metal carbonate, MCO3•M(OH)2, FB 1. (a) Method ● Weigh the empty crucible with its lid. Record the mass in the results section. ● Transfer all of the FB 1 from the container into the crucible. ● Weigh the crucible, lid and FB 1. Record the mass. ● Calculate the mass of FB 1 used. Record this mass in the space for other results. ● 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 5 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 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 the mass of residue obtained. Record this mass in the space for other results. Results mass of empty crucible and lid = .......................... mass of crucible, lid and FB 1 (before heating) = .......................... mass of crucible, lid and FB 1 (after first heating) = .......................... mass of crucible, lid and FB 1 (after second heating) = .......................... Other results I II III IV [4] (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 H2O) 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. Mr of MCO3•M(OH)2 = .............................. [1] (iv) Use your answer to (b)(iii) to calculate the relative atomic mass, Ar , of metal M. Show your working. Ar of M = .............................. [1] (c) (i) Explain why the headings for the third and fourth readings in the results section in (a) are not suitable. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) State whether or not your experiment would be more accurate if the crucible and its contents were heated for a third time. Explain your answer by referring to your results in (a). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) A student carries out the experiments in Questions 1 and 2. The student expects the value of the Mr of MCO3•M(OH)2 obtained by thermal decomposition in Question 2 to be more accurate than the value of the Mr obtained by titration in Question 1. State one reason why the student expects the experiment in Question 2 to be more accurate. ........................................................................................................................................... ..................................................................................................................................... [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. 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 Headings, units and precision of data  All four balance readings consistent to either 2 or 3 decimal places  unambiguous headings for (mass of) FB 1 and (mass of) residue / MO in the space provided  units given next to each balance reading and correctly displayed units for mass of FB 1 and mass of residue / MO II Quantities  fourth reading is within +0.02 and -0.05 g of third reading  mass of FB 1 used AND mass of residue correctly calculated  mass of FB 1 used within range 1.502.50 g 4 Accuracy marks mass ratio = mass FB 1 / mass of residue Calculate supervisor’s mass ratio to 2 d.p. Calculate candidate’s mass ratio to 2 d.p. Award III if candidate’s mass ratio is within 10% of the supervisor’s mass ratio. Award IV if candidate’s mass ratio is within 5% of the supervisor’s mass ratio. 2(b)(i) MCO3•M(OH)2(s) → 2MO(s) + CO2(g) + H2O(g) 1 2(b)(ii) Correctly calculates mol (CO2) = mass lost / (44 + 18) AND answer given to 2–4 significant figures 1 2(b)(iii) Correct use of (b)(ii) Mr = mass of FB 1 used / (b)(ii) AND answer given to 2–4 significant figures 1 Question Answer Marks 2(b)(iv) Correct expression and use to find Ar Ar = [(b)(iii) – (60 + 34)] / 2 1 2(c)(i) FB 1 has decomposed to metal oxide / residue is metal oxide 1 2(c)(ii) EITHER heating for a third time would not make it more accurate AND the fourth reading is equal (within 0.02 g) to the third OR heating for a third time would make it more accurate AND the fourth weighing is not the same as / lower than the third 1 2(c)(iii) One of the following reasons:  experiment 2 has fewer readings / steps / sources of error ORA OR examples of extra steps in experiment 1  experiment 2 end-point is easier to judge (because it is when mass is constant / experiment 1 end-point (colour change) is harder to judge)  burette / pipette has higher (percentage) error than balance 1

More questions on Reacting masses and volumes (of solutions and gases)

3 (a) FB 5 is a compound containing one cation and one anion, both of which are listed in the Qualitative analysis notes. (i) Heat a small spatula measure of FB 5 in a hard-glass test-tube until no further change occurs. Record your observations. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Describe another test to positively identify the cation in FB 5. Carry out your test and record your observations. test .................................................................................................................................... observations ...................................................................................................................... [1] (iii) Put a 1 cm depth of dilute hydrochloric acid in a test-tube. Add a small spatula measure of FB 5. Record your observations. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iv) Deduce the formula of FB 5. FB 5 is .............................. . [1] Question 3 continues on page 8. (b) You will devise chemical tests to distinguish between the two possible identities given for each of compounds FB 6, FB 7, FB 8 and FB 9. In each case you should: ● use a 1 cm depth of the solution of the unknown compound in a test-tube ● use a boiling tube if you need to warm a mixture ● use a spatula measure of the unknown solid ● record details of your test(s) and your observations ● state your conclusion about the identity of the compound. (i) FB 6 is either aqueous chromium(III) sulfate or aqueous iron(II) sulfate. FB 6 is ....................................................... . [2] (ii) FB 7 is either dilute hydrobromic acid or dilute nitric acid. If you select a test that gives a negative result, then you must carry out a further test that gives a positive result. FB 7 is ....................................................... . [2] (iii) FB 8 is either magnesium carbonate or zinc carbonate. FB 8 is ....................................................... . [2]

Mark scheme: 3(a)(i) (FB 5 is) white solid / powder or white / colourless crystals (at start)  condensation / steam produced  bullet 2 is linked to gentle heating / water vapour being produced immediately upon heating  no residue (at end) OR FB 5 / solid disappears / evaporates / sublimes (owtte)  attempts to test with litmus (paper)  (Gas / NH3) turns (moist red) litmus blue Two points needed for each mark. 2 3(a)(ii) warm FB 5 with (aqueous) NaOH AND gas / ammonia turns (red) litmus to blue 1 3(a)(iii)  fizzing / bubbles / effervescence (not ‘gas formed’)  colourless solution formed / remains or vigorous / rapid reaction (owtte)  attempts to test with lime water  gas / carbon dioxide gives white ppt / solid with lime water (not ‘milky’) Two points needed for each mark. 2 3(a)(iv) FB 5 is (NH4)2CO3 1 3(b) award M1 for suitable reagent to distinguish each pair award M2 for correct observation AND the conclusion 3(b)(i) M1 add NaOH or (acidified) KMnO4 / potassium manganate(VII) M2 (grey-)green ppt AND soluble / gives a (dark green) solution in excess (NaOH) OR (KMnO4) remains purple / (KMnO4) not decolourised AND FB 6 is chromium(III) sulfate / Cr2(SO4)3 2 3(b)(ii) M1 heat with (excess) NaOH AND Al M2 gas / NH3 turns litmus blue (after Al added) AND FB 7 is nitric acid / HNO3 2 Question Answer Marks 3(b)(iii) M1 dissolve / make a solution in a specified mineral acid AND add (aqueous) ammonia or (aqueous) NaOH M2 With NH3 / NaOH, gives white ppt AND soluble in excess AND FB 8 is zinc carbonate / ZnCO3 OR M1 heat the solid alone AND observe the hot residue / solid M2 (hot) residue is yellow AND FB 8 is zinc carbonate / ZnCO3 2 3(b)(iv) M1 add iodine AND (aqueous) NaOH M2 (pale) yellow / cream / off-white precipitate (on standing) AND FB 9 is ethanol / C2H5OH 2

More questions on Periodicity of chemical properties of the elements in Period 3