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Chemistry 9701/31 May/June 2020: question paper and mark scheme

Cambridge AS & A Level Chemistry (9701), Paper 3 Paper 31 (Advanced Practical Skills 1), May/June 2020. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.

9701_s20_qp_319701_s20_ms_319701/31/M/J/20

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Paper
Paper 3 Paper 31 (Advanced Practical Skills 1)
Total marks
40
Questions
3
Code on the paper
9701/31/M/J/20
9701_s20_ms_31.pdf

9701/31 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 118 marks

Question

In this experiment you will carry out a titration to determine the relative formula mass of a hydrated salt, FA 1. FA 1 is a hydrated salt. FA 2 is dilute sulfuric acid. FA 3 is 0.0200 mol dm–3 potassium manganate(VII). (a) Method Preparing a solution of FA 1 ●● Weigh the stoppered container of FA 1. Record the mass in the space below. ●● Tip all the FA 1 into the beaker. ●● Reweigh the container with its stopper. Record the mass. ●● Calculate and record the mass of FA 1 used. ●● Add approximately 100 cm3 of FA 2 to the FA 1 in the beaker. ●● Stir the mixture until all the FA 1 has dissolved. ●● Transfer this solution into the 250 cm3 volumetric flask. ●● Rinse the beaker and 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 the hydrated salt is FA 4. Label the flask FA 4. Titration ●● Fill the burette with FA 3. ●● ipette 25.0 cm3 of FA 4 into a conical flask. P ●● Use the 25.0 cm3 measuring cylinder to add 10 cm3 of FA 2 to the FA 4 in 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. I ●● Record in a suitable form below all of your burette readings and the volume of FA 3 added in each accurate titration. II Keep FA 3 and FA 4 for use in Question 3. III IV V VI VII VIII [8] (b) From your accurate titration results, obtain a suitable value for the volume of FA 3 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 4 required …… cm3 of FA 3. [1] (c) Calculations (i) Calculate the number of moles of potassium manganate(VII) present in the volume of FA 3 calculated in (b). moles of KMnO4 = …… mol [1] (ii) 1 mol of KMnO4 reacts with 5 mol of the hydrated salt, FA 1. Calculate the concentration of the hydrated salt, in mol dm–3, in FA 4. concentration of FA 4 = …… mol dm–3 [1] (iii) Use your answer to (c)(ii), and your data on page 2, to calculate an experimentally determined value for the relative formula mass of the hydrated salt, FA 1. Show your working. Mr of FA 1 = …… [1] [Total: 12] 2 In this experiment you will determine the enthalpy change of solution for anhydrous sodium carbonate. FA 5 is anhydrous sodium carbonate, Na2CO3. (You are given approximately 11 g.) (a) Method Experiment 1 ●● Weigh a cup. Record the mass. ●● Transfer 4.0– 4.2 g of FA 5 from the container into the cup. ●● Reweigh and record the mass of the cup with FA 5. ●● Calculate and record the mass of FA 5 used. ●● Support the cup in the 250 cm3 beaker. ●● Pour 30 cm3 of distilled water into the 50 cm3 measuring cylinder. ●● Measure and record the temperature of the distilled water in the measuring cylinder. ●● Add the 30 cm3 of distilled water to the FA 5 in the cup. ●● Stir constantly until the maximum temperature is reached. ●● Measure and record the maximum temperature. ●● Calculate and record the temperature rise. Experiment 2 ●● Repeat Experiment 1 but this time use 5.0–5.2 g of FA 5 and the other cup. ●● Record all data from both experiments in one table. I II III IV [4] (b) Calculations (i) Calculate the energy produced during Experiment 1. (Assume that 4.2 J change the temperature of 1.0 cm3 of solution by 1.0 °C.) energy produced = …… J [1] (ii) Calculate the number of moles of Na2CO3 used in Experiment 1. moles of Na2CO3 = …… mol [1] (iii) Use your answers to (b)(i) and (b)(ii) to calculate the enthalpy change, in kJ mol–1, for the reaction below. Show your working. Na2CO3(s) + aq → Na2CO3(aq) enthalpy change = …… …… kJ mol–1 sign value[1] (c) (i) A student suggested that by using the same thermometer, quantities of FA 5, and water, a more accurate value for the temperature rise could be calculated. Suggest how the student could obtain a more accurate measurement. . …… . …… . …… [1] (ii) State the maximum error in a single thermometer reading in your experiment in (a). maximum error = …… Hence calculate the maximum percentage error in the measurement of the temperature rise in Experiment 2. % error = …… [2] [Total: 10] 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. 3 (a) FA 6 is a hydrated salt. It contains two cations and one anion, all of which are listed in the Qualitative Analysis Notes. (i) Describe and carry out tests to identify the cations in FA 6. Record your tests and observations in the space below. The cations in FA 6 are …… and ���������������������������������������������� . [5] (ii) The anion in FA 6 is a sulfite, sulfate or a halide. Carry out a test to identify the anion in FA 6. Record your tests and observations in the space below. The anion in FA 6 is …… . [2] (iii) Give the ionic equation for one reaction you have carried out in (a)(i) or (a)(ii). Include state symbols. . …… [1] (iv) The formula of FA 6 is XY2Z2•wH2O where ●● X and Y are the cations present and Z is the anion present ●● w is the number of moles of water of crystallisation in the hydrated salt. The relative formula mass of this compound is 392.0. Using your conclusions from (a)(i) and (a)(ii), calculate the value of w, the number of moles of water of crystallisation. w = …… [2] (b) FA 7 and FA 8 are aqueous solutions of covalently bonded compounds. Half fill the beaker with water and place it on a tripod and gauze. Heat until the water begins to boil and then turn off the Bunsen burner. This will be used as a hot water bath. (i) Complete the table by carrying out the tests described. Use a 1 cm depth of FA 7 or FA 8 in a test-tube for each test. observation(s) test FA 7 FA 8 Test 1 Add an equal volume of dilute sulfuric acid and a few drops of FA 3, aqueous acidified potassium manganate(VII), then place in the hot water bath for several minutes. Test 2 Add an equal volume of dilute sulfuric acid and an equal volume of aqueous potassium iodide, then add a few drops of aqueous starch. Test 3 Add an equal volume of aqueous iodine, then add aqueous sodium hydroxide until no further change occurs. Leave the tube to stand. Test 4 Add a few drops of FA 4, then add aqueous ammonia. [5] (ii) FA 8 contains an organic compound. From your observation(s), suggest one possible identity for this compound. Explain your answer. name …… reason . …… . …… . …… [2] (iii) State the type of reagent FA 7 acts as in its reaction with aqueous potassium iodide. Explain your answer. . …… . …… . …… [1] [Total: 18] 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. 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

Mark scheme

1(a) I The following headings and data are recorded in the space provided 1 • mass of container with FA 1 • mass of (empty) container • mass of FA 1 used, correctly subtracted • consistent decimal places for weighings (at least one d.p.) II All the following data is recorded 1 • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations III Titre values recorded for accurate titrations, and Appropriate headings and units in the accurate titration table 1 • initial / start (burette) reading / volume • final / end (burette) reading / volume • titre or volume used / added / or FA 1 added (not ‘difference’ or ‘total’ or ‘amount’) • 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. 1 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 Award VI, VII and VIII if δ ⩽ 0.020 (cm3 g–1) 3 Award VI and VII if 0.020 < δ ⩽ 0.040 Award VI, only if 0.040 < δ ⩽ 0.060 1(b) Candidate calculates the mean correctly. 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 dp and be rounded to nearest 0.01 cm3. 1(c)(i) Correct calculation (expressed to 3 or 4 sf) 1 No of moles of KMnO4 used = 0.0200 × mean titre / 1000 The candidate’s mean titre must be used in the calculation 1(c)(ii) Correct use of (i) to calculate concentration of FA 4 1 Concentration of FA 4 = ans (i) × 5 × 1000 / 25 1(c)(iii) Correct expression for Mr 1 Mr = mass of FA 1 used × 4 / answer (ii) alternatively: Mr = ans (i) / 4

Question 2

Question

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–

Mark scheme

2(a) I Unambiguous table of data 1 • Mass of cup • Mass of cup + FA 5 • Mass of FA 5 used • Initial temperature or temperature of water / °C • Final / maximum temperature / °C • Temperature rise / °C • Both experiments must be attempted II Readings recorded appropriately 1 • All four thermometer readings recorded to .0 or .5. • All four subtractions correct (two masses and two temp rises). • No thermometer reading is less than 10 °C • Mass of FA 5 used for first experiment is 4.0–4.2 g • Mass of FA 5 used for second experiment is 5.0–5.2 g III + IV Accuracy marks Check and correct temp subtractions for candidate and supervisor. Round each measured temperature to 0.5 °C if necessary Compare the temperature rises for the two experiments. Award III if candidate’s rise is within 1.0 °C of supervisor in Expt 1. 1 Award IV if candidate’s rise within 1.0 °C of supervisor in Expt 2. 1 2(b)(i) Correct calculation of energy change (2, 3 or 4 sf) 1 Energy change = 30 × 4.2 × temp rise (J) 2(b)(ii) Correct calculation of no of moles of Na2CO3 used (2–4 sf) 1 n = mass claimed / 106 2(b)(iii) Correct use of (i) and (ii) to calculate enthalpy change (2–4 sf) 1 Enthalpy change = – energy / 1000 × moles 2(c)(i) Accuracy improvement 1 Plot graph to obtain better (estimate of) temp rise or plot a cooling curve 2(c)(ii) Error = 0.5 1 Correct calculation of % error to 2 or more sig fig 1 % error = 2 × 0.5 / temp rise (expt 2) × 100 FA 1/FA 6 is (NH4)2Fe(SO4)2.6H2O; FA 7 is H2O2; FA 8 is ethanol

Question 3

Question

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) Reagents used are NaOH and NH3 1 FA 6 dissolved in (distilled) water (before carrying out tests) 1 Observations with both cold alkalis 1 • With NaOH: green ppt, insoluble in excess • With NH3: green ppt, insoluble in excess OR • If only one of NaOH or NH3 was selected, award this mark if the observation is correct, but it must include ‘ppt turns brown’. Observation when heated with NaOH 1 Fizzing/bubbling and gas/NH3 turns (moist red) litmus to blue Both ions correctly identified 1 Iron(II) and ammonium (Fe2+ and NH4+) 3(a)(ii) Anion test and first observation 1 • Add barium nitrate/chloride • White precipitate Observation with acid and conclusion: 1 • white ppt is insoluble in specified mineral acid (not H2SO4) • sulfate / SO42– present 3(a)(iii) Ionic equation 1 Any one of the following equations, provided that the appropriate test was carried out. • Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s) • NH4+(aq) + OH–(aq) → NH3(g) + H2O(l or g) • Ba2+(aq) + SO42–(aq) → BaSO4(s) 3(a)(iv) Correct use of Mr to calculate no of moles water. 1 Mass of water = (392) - 55.8 -192.2 – 36 • n(H2O) = 392 – 284/18 (expressed as integer) 1 3(b)(i) Award one mark for every two correct observations (*) as shown in table below 5 test observation(s) FA 5 FA 6 Test 1 KMnO4 decolorised*. no reaction KMnO4 not decolorised fizzing/bubbling/effervescence or gas relights glowing spill* and ......................................................................... ......................................................................... No (further) change KMnO4 goes from purple to colourless (solution)* Test 2 Red-brown/ brown solution formed* No change and ......................................................................... ......................................................................... Mixture goes to (dark) blue / blue-black* No change* Test 3 Yellow or colourless solution formed* (On standing) off-white / pale yellow precipitate formed*. Test 4 No change / pale yellow (solution) formed* ......................................................................... Red-brown / brown / rust precipitate (formed)* Ignore fizzing 3(b)(ii) FA 8 is ethanol or propan-2-ol or butan-2-ol (or any secondary 2-ol) 1 Correct reference to the CHI3 test or to CH3CH(OH)- or CH3CO group Correct reference to the redox reaction with KMnO4 (provided that FA 8 was identified as an alcohol/aldehyde). 1 3(b)(iii) FA 7 is an oxidising agent because iodine is formed. 1

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Chemistry 9701/31 for May/June 2020 is out of 40 marks, across 3 questions.

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