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Chemistry 9701/33 October/November 2021: question paper and mark scheme

Cambridge AS & A Level Chemistry (9701), Paper 3 Advanced Practical Skills 1, October/November 2021. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.

9701_w21_qp_339701_w21_ms_339701/33/O/N/21

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Paper
Paper 3 Advanced Practical Skills 1
Total marks
40
Questions
3
Code on the paper
9701/33/O/N/21
9701_w21_ms_33.pdf

9701/33 mark scheme, question by question

Each question as it's printed, and the mark scheme's points for it, as text. Diagrams, tables and graphs are in the PDFs above.

Question 110 marks

Question

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 1 • (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 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 mass FA 1 1 IV Examiner calculates ratio to 2 dp for candidate and difference from supervisor ratio. mass carbon dioxide The mark is awarded for a difference ⩽ 0.25. 1(b)(i) Correctly calculates 1 amount of CO2 = mass CO2 lost / 44 mol AND answer given to 2 – 4 significant figures 1(b)(ii) Correctly uses 1 mass FA 1 Mr = (b)(i) AND answer given to 2 – 4 sf (sf error is not penalised more than once in 1(b)) 1(b)(iii) Correct expression 1 ((b)(ii) − 60) Ar M+ = 2 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(c)(i) Any one of: 1 • 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(c)(ii) M1: amount of acid added to flask, n(H+) = 0.05 mol 2 correct working or answer OR 2 × (b)(i) M2: n(H+) > 2 × n(M2CO3) mass FA 1 where n(M2CO3) = (b)(i) or (b)(ii) Other valid methods may be awarded.

Question 214 marks

Question

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: 1 • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings. II Headings and units correct for accurate titration table and headings match readings. 1 • initial / start AND (burette) reading / volume + unit • final / end AND (burette) reading / volume + unit • titre OR volume / FA 4 AND used / added + unit 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 3 values. Award V if δ ⩽ 0.60 cm3 Award VI if δ ⩽ 0.40 cm3 Award VII if δ ⩽ 0.20 cm3 2(b) Candidate must average two (or more) titres that are all within 0.20 cm3 1 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. 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 1 n(HCl ) = 0.110 × (b) / 1000 2(c)(iii) (c)(ii) 1 Correctly uses n(M2CO3) in 25 cm3 = 2 2(c)(iv) 7.46 1 Correctly uses Mr = (c)(iii) × 40 2(c)(v) ((c)(iv) − 60) 1 Correctly uses 2 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 + 2(d) Acid was more concentrated because smaller titre gives fewer moles of M2CO3 1 OR specimen calculation showing effect of different concentration provided. (titre ↓ = > n(M2CO3) ↓ = > Mr↑) ORA FA 5 is (NH4)2CO3(s); FA 7 is CrK(SO4)2(aq); FA 8 is BaCl 2(aq)

Question 316 marks

Question

(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] Qualitative Analysis Notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. pale blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess 2 Reactions of anions ion reaction carbonate, CO2 liberated by dilute acids CO3 2– chloride, gives white ppt. with Ag+(aq) (soluble in NH3(aq)) Cl –(aq) bromide, gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)) Br (aq) – iodide, gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) I –(aq) nitrate, NH3 liberated on heating with OH–(aq) and Al foil NO3–(aq) nitrite, NH3 liberated on heating with OH–(aq) and Al foil NO2–(aq) sulfate, gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids) SO42–(aq) sulfite, gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) SO3 (aq)2– 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2) chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint The Periodic Table of Elements Group 1 2 13 14 15 16 17 18 1 2 H He hydrogen helium Key 1.0 4.0 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 23.0 24.3 3 4 5 6 7 8 9 10 11 12 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 at www.cambridgeinternational.org after the live examination series. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 12 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 114 116 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Fl Lv francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium flerovium livermorium – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 138.9 140.1 140.9 144.4 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232.0 231.0 238.0 – – – – – – – – – – – Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge

Mark scheme

3(a)(i) Award any two 2 • 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 1 AND forms a white ppt with limewater/ limewater turns milky / chalky / cloudy white 3(b)(i) M1, M2, M3 and M4 from observations table 4 2 * = 1 mark (round down) FA 6 FA 7 FA 8 + Ag+ white ppt * no (visible) reaction / no change / no white ppt * (insoluble in excess) ppt / solution remains blue / green / (insoluble in excess) blue-green * + FA 6 effervescence / fizzing / bubbles * white ppt * (blue) solution turns green / greener * (insoluble in excess) grey-blue / grey-green / (pale) green ppt * (award a positive limewater test in (a)(ii)) + FA 7 white ppt * (insoluble in excess) 3(b)(ii) M1: Tabulates the two reagents and observations for FA 6, FA 7 and FA 8 in clearly labelled sections. 5 M2, M3, M4 and M5: from observations table 2 * = 1 mark (round down) FA 6 FA 7 FA 8 + NaOH no (visible) reaction / no grey-green ppt * soluble in (faint) white ppt change / no ppt / remains excess / (dark) green solution and insoluble in excess * colourless solution * formed with excess * (allow no change / no (visible) reaction / no ppt) + warm * gas / NH3 turns (red) litmus If (red) litmus turning blue is reported for either FA 7 or FA 8 then blue * gas / NH3 turns (red) litmus blue with FA 6 cannot be credited. + H2SO4 Fizzing / bubbling/ no (visible) reaction / no change / white ppt * effervescence * no ppt * (insoluble in excess) (award a positive limewater test in (a)(ii)) 3(c)(i) M1, M2 and M3 from conclusions table: 3 2 * = 1 mark (round down) cation anion FA 6 NH4+ * CO32– * FA 7 Cr3+ * SO42– * FA 8 Ba2+ / Ca2+ * Cl – * 3(c)(ii) Correct ionic equation from test in (a) or (b) and ion given in (c)(i) 1 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)

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Chemistry 9701/33 for October/November 2021 is out of 40 marks, across 3 questions.

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