Cambridge A Level Chemistry 9701 — 2021 Oct/Nov Paper 3 · Variant 3

1 Group 1 metal carbonates have the formula M2CO3. The identity of the metal ion, M+, may be determined by a gravimetric method. The metal carbonate is reacted with excess acid and the mass of carbon dioxide given off is measured. M2CO3(s) + 2HCl (aq) → 2MCl (aq) + H2O(l) + CO2(g) FA 1 is a Group 1 metal carbonate, M2CO3. FA 2 is 2.0 mol dm–3 hydrochloric acid, HCl. (a) Method ● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of FA 2 into a conical flask. Weigh the flask with the acid and record the mass. ● Weigh the container with FA 1 and record the mass. ● Carefully tip all of FA 1 into the acid in the conical flask. Swirl the contents of the flask and leave the flask to stand. ● Weigh the container with any residual FA 1. Record the mass. ● Calculate and record the mass of FA 1 added to the conical flask. ● Calculate and record the theoretical initial mass of flask + acid + FA 1. ● Swirl the flask occasionally while leaving it to stand for approximately 5 minutes. During this step you may wish to start Question 2 or Question 3. ● Weigh the flask and contents and record this mass. ● Calculate and record the mass of carbon dioxide given off during the experiment. Results I II III IV [4] (b) Calculations (i) Calculate the number of moles of carbon dioxide given off in the experiment. moles of CO2 = .............................. mol [1] (ii) Calculate the relative formula mass, Mr, of M2CO3. Mr of M2CO3 = .............................. [1] (iii) Identify the Group 1 cation, M+, in FA 1. Show your working. M+ is .............................. . [1] (c) One source of error in this experiment is the solubility of carbon dioxide in water. (i) Suggest one modification, to the method in (a), to reduce the solubility of carbon dioxide in the solution in the flask. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) An assumption made in the method in (a) is that the acid is in excess. Show by calculation that this assumption is true. [2] [Total: 10]

Mark scheme: 1(a) I Unambiguous headings AND correct units in the space provided • (mass) flask + acid / FA 2 • (mass) container + M2CO3 / FA 1 • (mass) container (+ residue) • (mass) flask + contents (provided this label not used for theoretical mass) • (mass) FA 1 (added) • theoretical / total mass or flask + FA 2 / acid + FA 1 / M2CO3 • (mass) CO2 1 II Four balance readings recorded to the same number of decimal places AND answer to at least 1 decimal place 1 III Three masses correctly calculated. 1 IV Examiner calculates ratio mass mass carbon dioxide FA 1 to 2 dp for candidate and difference from supervisor ratio. The mark is awarded for a difference ⩽ 0.25. 1 1(b)(i) Correctly calculates amount of CO2 = mass CO2 lost / 44 mol AND answer given to 2 – 4 significant figures 1 1(b)(ii) Correctly uses Mr = mass FA 1 (b)(i) AND answer given to 2 – 4 sf (sf error is not penalised more than once in 1(b)) 1 Question Answer Marks 1(b)(iii) Correct expression Ar M+ = − ( 60) 2 (b)(ii) AND M+ is the closest group 1 cation 0.1 < Li+ ⩽ 14.9 15.0 ⩽ Na+ ⩽ 31.1 31.2 ⩽ K+ ⩽ 62.3 62.3 ⩽ Rb+ ⩽ 109.2 109.2 ⩽ Cs+ ⩽ 250 1 1(c)(i) Any one of: • saturate the acid with CO2 initially • pre-heat the acid • use more concentrated acid • shake / swirl the flask vigorously / more • add named inert solute to solution 1 1(c)(ii) M1: amount of acid added to flask, n(H+) = 0.05 mol correct working or answer OR 2 × (b)(i) M2: n(H+) > 2 × n(M2CO3) where n(M2CO3) = (b)(i) or mass FA 1 (b)(ii) Other valid methods may be awarded. 2

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2 The identity of a Group 1 metal carbonate may also be found by a titration method. M+ in this question may or may not be the same cation as that in Question 1. M2CO3(s) + 2HCl (aq) → 2MCl (aq) + H2O(l) + CO2(g) FA 3 is an aqueous solution containing 7.46 g dm–3 of a Group 1 metal carbonate, M2CO3. FA 4 is 0.110 mol dm–3 hydrochloric acid, HCl. 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 bromophenol blue indicator. ● Carry out 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 accuracy of your practical work. ● Record, in a suitable form below, all your burette readings and the volume of FA 4 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 obtained this value. 25.0 cm3 of FA 3 required .............................. cm3 of FA 4. [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 number of moles of hydrochloric acid present in the volume of FA 4 calculated in (b). moles of HCl = .............................. mol [1] (iii) Calculate the number of moles of Group 1 metal carbonate, M2CO3, present in 25.0 cm3 of FA 3. moles of M2CO3 in 25.0 cm3 = .............................. mol [1] (iv) Calculate the relative formula mass, Mr, of M2CO3. Mr of M2CO3 = .............................. [1] (v) Identify the cation, M+. Show your working. M+ is .............................. . [1] (d) A student carrying out a similar experiment, using the same method, found the cation in Question 2 to be Rb+. The student is told that the acid provided, FA 4, was incorrectly prepared. The cation in the student’s experiment should have been identified as K+. State whether the acid supplied is more, or less, concentrated than 0.110 mol dm–3. Explain your answer. .................................................................................................................................................... .................................................................................................................................................... [1] [Total: 14] Qualitative analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen ● the formation of any precipitate and its solubility in an excess of the reagent added ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted.

Mark scheme: 2(a) I The following data must be shown: • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings. 1 II Headings and units correct for accurate titration table and headings match readings. • initial / start AND (burette) reading / volume + unit • final / end AND (burette) reading / volume + unit • titre OR volume / FA 4 AND used / added + unit 1 III All accurate burette readings given to the nearest 0.05 cm3 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Examiner calculates the mean titres for supervisor and candidate and awards marks according to the difference, d, in the values. Award V if δ ⩽ 0.60 cm3 Award VI if δ ⩽ 0.40 cm3 Award VII if δ ⩽ 0.20 cm3 3 2(b) Candidate must average two (or more) titres that are all within 0.20 cm3 AND give the answer to 2 dp. AND working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1 2(c)(i) All answers for (c)(ii), (c)(iii), (c)(iv) are to 3 – 4 sf 1 2(c)(ii) Correctly calculates amount of acid n(HCl ) = 0.110 × (b) / 1000 1 Question Answer Marks 2(c)(iii) Correctly uses n(M2CO3) in 25 cm3 = 2 (c)(ii) 1 2(c)(iv) Correctly uses Mr = × 7.46 40 (c)(iii) 1 2(c)(v) Correctly uses − ( 60) 2 (c)(iv) AND M+ is the closest group 1 cation 0.1 < Li+ ⩽ 14.9 15.0 ⩽ Na+ ⩽ 31.1 31.2 ⩽ K+ ⩽ 62.3 62.3 ⩽ Rb+ ⩽ 109.2 109.2 ⩽ Cs+ ⩽ 250 1 2(d) Acid was more concentrated because smaller titre gives fewer moles of M2CO3 OR specimen calculation showing effect of different concentration provided. (titre ↓ = > n(M2CO3) ↓ = > Mr↑) ORA 1

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3 (a) FA 5 is a salt containing one cation and one anion, both of which are listed in the Qualitative Analysis Notes. (i) Place a small spatula measure of FA 5 into a hard‑glass test‑tube and heat the tube, gently at first and then more strongly. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Place the remaining FA 5 into a 100 cm3 beaker and add approximately 15 cm3 of distilled water. Stir to make a solution. This solution is FA 6. You will use portions of FA 6 for the following test and tests in (b). To a 1 cm depth of FA 6 in a test‑tube add a 1 cm depth of dilute hydrochloric acid. Record your observations. ............................................................................................................................................. ....................................................................................................................................... [1] (b) (i) FA 7 and FA 8 are solutions each containing one cation and one anion, all of which are listed in the Qualitative Analysis Notes. Carry out the following tests in separate test‑tubes. Use a 1 cm depth of each solution unless otherwise specified. observations solution FA 6 FA 7 FA 8 Add a few drops of aqueous silver nitrate. FA 6 FA 7 [4] (ii) Carry out tests using aqueous sodium hydroxide and dilute sulfuric acid to identify or confirm the identity of the ions in FA 6, FA 7 and FA 8. Record your tests and observations in a table in the space below. [5] (c) (i) From your observations in (a) and (b) identify the cation and the anion present in each of FA 6, FA 7 and FA 8 by giving their formulae. cation anion FA 6 FA 7 FA 8 [3] (ii) Give an ionic equation for a precipitation reaction observed in (b)(i). Include state symbols. ....................................................................................................................................... [1] [Total: 16]

Mark scheme: 3(a)(i) Award any two • Sublimes / solid diminishes / disappears / solid formed on cooler part of tube / no residue • Condensation / water droplets / steam • Gas / NH3 turns (damp red) litmus blue. A positive test with limewater is credited in (a)(ii) 3(a)(ii) Effervescence / bubbling / fizzing AND forms a white ppt with limewater/ limewater turns milky / chalky / cloudy white 1 3(b)(i) M1, M2, M3 and M4 from observations table 2 * = 1 mark (round down) FA 6 FA 7 FA 8 + Ag+ white ppt * (insoluble in excess) no (visible) reaction / no change / no ppt / solution remains blue / green / blue-green * white ppt * (insoluble in excess) + FA 6 effervescence / fizzing / bubbles * (blue) solution turns green / greener * grey-blue / grey-green / (pale) green ppt * (award a positive limewater test in (a)(ii)) white ppt * (insoluble in excess) + FA 7 white ppt * (insoluble in excess) 4 Question Answer Marks 3(b)(ii) M1: Tabulates the two reagents and observations for FA 6, FA 7 and FA 8 in clearly labelled sections. M2, M3, M4 and M5: from observations table 2 * = 1 mark (round down) FA 6 FA 7 FA 8 + NaOH no (visible) reaction / no change / no ppt / remains colourless solution * grey-green ppt * soluble in excess / (dark) green solution formed with excess * (faint) white ppt and insoluble in excess * (allow no change / no (visible) reaction / no ppt) + warm * gas / NH3 turns (red) litmus blue * If (red) litmus turning blue is reported for either FA 7 or FA 8 then gas / NH3 turns (red) litmus blue with FA 6 cannot be credited. + H2SO4 Fizzing / bubbling/ effervescence * (award a positive limewater test in (a)(ii)) no (visible) reaction / no change / no ppt * white ppt * (insoluble in excess) 5 3(c)(i) M1, M2 and M3 from conclusions table: 2 * = 1 mark (round down) cation anion FA 6 NH4+ * CO32– * FA 7 Cr3+ * SO42– * FA 8 Ba2+ / Ca2+ * Cl – * 3 Question Answer Marks 3(c)(ii) Correct ionic equation from test in (a) or (b) and ion given in (c)(i) Examples of correct ionic equations: Ba2+(aq) + SO42−(aq) → BaSO4(s) Ba2+(aq) + CO32−(aq) → BaCO3(s) Ba2+(aq) + 2OH−(aq) → Ba(OH)2(s) Ag+(aq) + Cl –(aq) → AgCl (s) 2Ag+(aq) + CO32−(aq) → Ag2CO3(s) Ca2+(aq) + CO32−(aq) → CaCO3(s) 1

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