Free until 14 Nov · 80% off for the first 100 · Pricing

Get started

Chemistry 9701/31 May/June 2021: question paper and mark scheme

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

9701_s21_qp_319701_s21_ms_319701/31/M/J/21

Mark my answers with AIRead the paper and scheme
Paper
Paper 3 (Advanced Practical Skills 1)
Total marks
40
Questions
3
Code on the paper
9701/31/M/J/21
9701_s21_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 116 marks

Question

Iron(II) sulfate crystals, FeSO4•xH2O, contain water of crystallisation. You will carry out a titration to determine the value of x in the formula, where x is an integer. A solution containing a known mass of the crystals will be titrated with acidified aqueous potassium manganate(VII) of known concentration. 5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) FA 1 contains 26.52 g dm–3 of hydrated iron(II) sulfate, FeSO4•xH2O. FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is dilute sulfuric acid, H2SO4. (a) Method ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 1 into a conical flask. ● Use the 25 cm3 measuring cylinder to transfer 25 cm3 of FA 3 into the same conical flask. ● 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 2 added in each accurate titration. Keep FA 1 for use in Question 3. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 1 required …… cm3 of FA 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 number of moles of potassium manganate(VII) present in the volume of FA 2 calculated in (b). moles of KMnO4 = …… mol [1] (iii) Calculate the number of moles of iron(II) sulfate present in 1.00 dm3 of FA 1. moles of FeSO4 = …… mol [1] (iv) Calculate the mass of iron(II) sulfate present in 1.00 dm3 of FA 1. mass of FeSO4 = …… g [1] (v) Calculate the value of x in FeSO4•xH2O. x = …… [2] (d) Iron(II) sulfate in solution is readily oxidised by air to form iron(III) sulfate. State the effect, on the value of x calculated in (c)(v), if some of your sample of FA 1 had oxidised before you carried out the titration. Explain your answer. . …… . …… . …… . …… [2] [Total: 16] Question 2 starts on the next page.

Mark scheme

1(a) I The following data must be shown 1 • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings ‘Correct’ headings and units are not required for this mark II Headings and units correct for accurate titration table and headings match readings. 1 • initial/start and (burette) reading / volume + unit (allow vol but not V) • final/end and (burette) reading / volume + unit (allow value for reading) • titre or volume/FA 2 and used/added (not ‘difference’ or ‘total’ or ‘amount’) + unit Units: (cm3) or / cm3 or in cm3 or cm3 by every entry III All accurate burette readings to 0.05 cm3 1 Do not award this mark if: 50(.00) is used as an initial burette reading; More than one final burette reading is 50(.00); Any burette reading is greater than 50(.00). IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Do not award the mark if any ‘accurate’ burette readings (apart from initial 0) are given to zero dp. For assessment of accuracy (Q) marks, the Examiner should round any burette readings to the nearest 0.05 cm3. Check 1 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 difference (δ) between the candidate’s titre and the supervisor’s titre. Award the accuracy (Q) marks as shown below. Award V if δ ⩽ 0.50 cm3 1 Award VI if δ ⩽ 0.30 cm3 1 Award VII if δ ⩽ 0.20 cm3 1(b) Candidate must average two (or more) titres that are all within 0.20 cm3. 1 Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1(c)(i) Answers for (c)(ii), (c)(iii), (c)(iv) to 3–4 sf 1 1(c)(ii) Correctly calculates 1 n(MnO4−) = 0.02 × (b) / 1000 1(c)(iii) Correctly uses 1 n(FeSO4) in 25 cm3 = (c)(ii) × 5 × 40 1(c)(iv) Correctly uses 1 mass(FeSO4) in 1 dm3 = (c)(iii) × 151.9 1(c)(v) Correctly uses 1 n(H2O) = [26.52 – (c)(iv)] / 18 ratio n(H2O) : n((FeSO4) 1 AND x given as integer OR M1: Mr hydrated salt = 26.52 / (c)(iii) M2: (26.52 / (c)(iii) – 151.9) / 18 and answer as integer 1(d) (as) mole / amount / volume MnO4− smaller 1 or (as) mass / moles / amount Fe2+ smaller mass/moles/amount water larger so (mole) ratio larger so x increases 1 Allow difference in titre/moles of MnO4− / Fe2+ will be too small to change the (integer) value of x for 1 mark. Allow for 1 mark: less MnO4− / Fe2+ and x increases

Question 28 marks

Question

In Question 1 you used a titration method to determine the value of x in a hydrated salt. In Question 2 you will use a gravimetric method. In this method a sample of solid is heated to remove the water of crystallisation. You will carry out this method on a different hydrated compound, FA 4, with formula MZ•yH2O. In FA 4 the value of y is an integer. MZ•yH2O(s) → MZ(s) + yH2O(g) FA 4 is a hydrated compound, MZ•yH2O. (a) Method ● Weigh the crucible with its lid. Record the mass. ● Place between 2.40 g and 2.60 g of FA 4 in the crucible and record its appearance below. ● Weigh the crucible, its lid and contents and record the mass. ● Without the lid, place the crucible on the pipe-clay triangle and heat gently for approximately one minute and record your observations. ● Then heat more strongly for approximately four minutes. ● Place the lid on the crucible and leave it to cool. You may wish to start Question 3 while you are waiting for the crucible to cool. ● Weigh the crucible, its lid and contents and record the mass. ● Calculate and record the mass of FA 4, the mass of residue after heating and the mass of water lost. Keep FA 4 for use in Question 3. Results appearance of FA 4 …… observations during heating for the first minute . …… . …… I II III IV [4] (b) Calculations (i) Calculate the number of moles of water lost when your sample of MZ•yH2O was heated. moles of water = …… mol The relative formula mass of the anhydrous compound MZ is 120.4. Calculate the number of moles of MZ present in the residue. moles of MZ = …… mol [1] (ii) Use your answers from (b)(i) to calculate the value of y in FA 4, MZ•yH2O. Show your working. y = …… [1] (iii) State an assumption you made when calculating the value of y in the hydrated compound. . …… [1] (c) A student suggested that the experiment would be more accurate if the crucible had been heated with the lid on for the first minute. State and explain whether you agree with the student. . …… . …… [1] [Total: 8] 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 Appearance 1 FA 4: white / colourless and crystals /solid AND obs during heating: swelling / steam / (some) solid melts / dissolves / white powder forms around the edges Allow evaporation of water Allow bubbling / bubbles of water Ignore fizzing / effervescence II Unambiguous headings and units in list / table of data: 1 • (Mass of) crucible and lid (not ‘weight’ in any reading) • (Mass of) crucible, lid and FA 4 (or ‘contents before heating’) • (Mass of) crucible, lid and residue / contents after heating • (Mass of) FA 4 (used) • (Mass of) residue • (Mass of) water (lost) Ignore mention of 1st heating if reheated Do not allow ‘mass of FA 4 after heating’ in third weighing. Use of lid must be consistent. Unit must be given correctly in each case. III Three weighings recorded and all listed subtractions (minimum one) correct 1 • All weighings to the same number of dp • Mass of FA 4 used between 2.40 and 2.60 g If 4 weighings shown then all must be to the same number of dp IV Accuracy mark 1 Award if ratio between 1.80–2.10 2(b)(i) Correctly calculates 1 n(H2O) = mass loss in (a) / 18 and answer to 2 or more sf AND n(MZ) = mass of residue in (a) / 120.4 and answer to 2 or more sf If masses are not shown in (a) then values used must be correct 2(b)(ii) Shows use of 1 ratio n(MZ) : n(H2O) AND y is an integer Other correct methods are available. 2(b)(iii) One of the following 1 • All water (of crystallisation) was lost / dehydration was complete • No MZ decomposed on heating 2(c) One of the following 1 • student incorrect and because there was no spitting (and therefore lid not required) • student correct and to catch any spitting (check observations for spitting or swelling) • student incorrect and because putting the lid on makes no difference to water (vapour) escaping

Question 316 marks

Question

(a) Aqueous ammonium thiocyanate reacts with aqueous iron(III) ions to form an orange or red coloured compound. Iron(II) ions do not react in this way. The darker the orange or red colour, the more iron(III) ions are present in the solution. (i) For each test use a 1 cm depth of FA 1 in a test‑tube. Record all your observations. test observations Test 1 Add a few drops of aqueous ammonium thiocyanate. Test 2 Add a few drops of aqueous sodium hydroxide and leave for at least two minutes, then add dilute sulfuric acid dropwise until there is no further change, then add a few drops of aqueous ammonium thiocyanate. [3] (ii) Suggest a reason for any difference in observation when you added aqueous ammonium thiocyanate in Test 2 compared with Test 1. Your answer should refer to the type of reaction that occurred in Test 2. . …… . …… . …… [2] (iii) The charge on the thiocyanate ion, SCN–, is –1. Determine the formula of ammonium thiocyanate. . …… [1] (iv) A solution containing Fe2+ reacts with aqueous ammonia to form a green precipitate. Write the ionic equation for this reaction. Include state symbols. . …… [2] (b) FA 4 contains one cation and one anion, both of which are listed in the Qualitative Analysis Notes. The anion in FA 4 contains sulfur. (i) Carry out appropriate tests to allow you to identify the cation and anion in FA 4. Record each test and your observations in a suitable form below. [7] (ii) Give the formula of the ions present in FA 4. cation …… anion . …… [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 CO32– 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) SO4 (aq) 2– 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 16 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) Test 1 no change / (pale) orange / (pale) red / (pale) pink solution 1 Allow solution becomes colourless / paler Test 2 2 + NaOH green / dirty green / pale green / dark green ppt * ppt turns brown (at surface) * + H2SO4 ppt dissolves or yellow / yellow-brown / orange-brown solution formed * + SCN− (solution) turns dark(er) orange / blood-red / red / dark(er) red / deep red / red-brown * colour must be more intense than in Test 1 2 asterisks = 1 mark (round down) 3(a)(ii) Fe3+ is formed (in Test 2) 1 oxidation of Fe2+ / redox 1 3(a)(iii) NH4SCN 1 3(a)(iv) Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s) correct product formula 1 balancing and state symbols 1 3(b)(i) prepare solution of FA 4 1 prepare a suitable table(s) 1 minimum 2 tests and columns/ rows for tests and for observations 2 asterisks = 1 mark (round down) 5 +NaOH * white ppt * insoluble in excess * + NH3 * white ppt* insoluble in excess * BaCl2/Ba(NO3)2* white ppt * HCl / HNO3 * insoluble in excess suitable named acid * OR no change / no reaction / solution remains dilute acid * + purple ** KMnO4 * Ignore additional reagents 3(b)(ii) cation: Mg2+ 1 and anion: SO42−

Checked your answers against these? Let AI mark the whole paper, on your own working.

Mark my 9701/31

How this paper is marked

Examiners follow the generic marking principles at the front of the scheme, then each question's points: method (M), accuracy (A) and independent (B) marks, error carried forward where the scheme allows it, and positive marking, so you're credited for what you got right.

Check your answers against it

Done this paper? Upload your script and CIE Buddy's AI marker marks it question by question against this exact scheme, puts the ticks on your own working and says what each lost mark needed.

Other variants, May/June 2021

9701/31 in other series

Find another Chemistry paper

Every AS & A Level Chemistry mark scheme

Question papers and mark schemes © Cambridge University Press & Assessment. CIE Buddy is independent and not connected to Cambridge.

9701/31 questions

Where can I find the 9701/31 May/June 2021 mark scheme?

On this page, free: the mark scheme for AS & A Level Chemistry 9701/31, May/June 2021 (9701_s21_ms_31), and the question paper (9701_s21_qp_31), to read or download, with each question's marking points written out underneath.

How many marks is 9701/31 out of?

Chemistry 9701/31 for May/June 2021 is out of 40 marks, across 3 questions.

Can I get my 9701/31 answers marked?

Yes. Upload your answers to 9701/31/M/J/21 as a PDF or photos, and CIE Buddy marks every question against this paper's official mark scheme, using Cambridge's marking rules. You get your score, each lost mark explained on your own script, and the topics to revise. It's a guide, not an official Cambridge grade.