Chemistry 9701/33 October/November 2022: question paper and mark scheme

Question

An iron compound can contain Fe2+ ions, Fe3+ ions or both ions. In this experiment, you will determine the percentage by mass of iron in FA 1, an unknown compound. You will first prepare a solution of the compound and then carry out a titration using acidified potassium manganate(VII), KMnO4. 5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) The end-point of the titration is when all the Fe2+ ions have been oxidised and so unreacted potassium manganate(VII) causes the colour of the solution to become a permanent pale pink. FA 1 is a sample of the unknown compound. FA 2 is 0.0100 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is dilute sulfuric acid, H2SO4. (a) Method Weighing the compound ● Weigh the sample of FA 1 and its container. ● Tip the FA 1 into the 250 cm3 beaker. ● Reweigh the container including any residual FA 1. ● Record both your readings clearly in the space below. ● Calculate the mass of FA 1 transferred into the beaker. Record the mass. Preparing the solution ● Add approximately 200 cm3 of distilled water to the beaker and stir until the FA 1 has dissolved. ● Pour the contents carefully into the 250 cm3 volumetric flask. ● Rinse the contents of the beaker with a little distilled water and add these washings to the flask. ● Fill the flask to the line with distilled water and shake thoroughly. ● Label this solution FA 4. Titration ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 4 into a conical flask. ● Use the 25 cm3 measuring cylinder to add 15 cm3 of FA 3 to the conical flask. ● Add FA 2 from the burette until the solution in the flask turns a permanent pink. ● Carry out a rough titration and record your burette readings in the space below. rough titre = …… cm3 ● Carry out as many accurate titrations as you think are necessary to obtain consistent results. ● Make sure that your recorded results show the precision of your practical work. ● Record in a suitable format, in the space below, all your burette readings and the volume of FA 2 added in each accurate titration. I II III IV V VI VII VIII [8] (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 4 required …… cm3 of FA 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to the appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of manganate(VII) ions in the volume recorded in (b). amount of MnO4– = …… mol [1] (iii) Calculate the amount, in mol, of iron(II) ions in the weighed sample of FA 1. amount of Fe2+ = …… mol [1] (iv) Calculate the percentage by mass of iron in FA 1. Show your working. percentage by mass of iron = …… % [2] (d) A student carries out the same experiment as in (a). The student receives a sample of FA 1 in a container with a lid. The student records the initial mass of the container with its lid and the sample of FA 1. Then the student records the mass of the container with the residue but forgets to replace the lid. How would this error alter the student’s answer to (c)(iv)? Explain your answer. . …… . …… [1] (e) State two assumptions that have been made in calculating the percentage by mass of iron in FA 1 in (c)(iv). . …… . …… . …… [2] [Total: 17] Question 2 starts on the next page.

Question

In this experiment you will determine the percentage by mass of magnesium in a hydrated salt, MgX•7H2O, where X represents the anion. You will measure the loss of mass when a sample of the hydrated salt is heated to form the anhydrous salt. FA 5 is a pure sample of MgX•7H2O (a) Method ● Weigh the crucible with its lid. Record the mass. ● Add all of FA 5 to the crucible. ● Reweigh the crucible with its lid and FA 5. Record the mass. ● Support the crucible in the pipe-clay triangle on top of the tripod. ● Heat the crucible with the lid on gently for about 1 minute. ● Remove the lid and then heat strongly for a further 4 minutes. ● Replace the lid and allow the crucible to cool for at least 5 minutes. While the crucible is cooling you may wish to begin work on Question 3. ● When the crucible is cool enough to handle, reweigh the crucible with its lid and its contents. Record the mass. ● Calculate and record the mass of FA 5 used. ● Calculate and record the mass of water lost. I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of water lost. amount of H2O lost = …… mol [1] (ii) Calculate the percentage by mass of magnesium in FA 5. Show your working. percentage by mass of Mg = …… % [2] (c) Suggest two assumptions that must be made for this experiment to give an accurate value of the percentage of Mg. . …… . …… . …… . …… [2] [Total: 10] 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. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Question

(a) FA 6 is an acidified aqueous solution of a salt which contains two cations and one anion, all of which are listed in the Qualitative analysis notes. (i) Select a reagent or reagents for use in tests to identify the two cations. Record your observations. [2] (ii) The anion in FA 6 is either the sulfate ion, SO42–, or the sulfite ion, SO32–. Select a reagent or reagents for use in tests to identify the anion. Record your observations. [2] (iii) Identify the ions present in FA 6 and give their formulae. The cations present in FA 6 are …… and …… . The anion present in FA 6 is …… . [2] (b) Carry out the tests and record your observations. For each test use a 1 cm depth of FA 6 in a test-tube. test observations Test 1 Add a 1 cm depth of aqueous sodium sulfite, then add a 1 cm depth of dilute sulfuric acid. Rinse out this test-tube thoroughly. Test 2 Add a 1 cm depth of aqueous potassium iodide, then add a few drops of starch solution. [2] (c) FA 7, FA 8 and FA 9 are aqueous solutions of sodium salts. Each solution contains one of the following ions: CO32–, Cl –, Br –. You will identify which solution contains which ion. (i) Carry out the tests and record your observations. For each test use a 1 cm depth of FA 7, FA 8 or FA 9 in a test-tube. observations test FA 7 FA 8 FA 9 Add a few drops of aqueous silver nitrate, then add aqueous ammonia. [3] (ii) If your results are insufficient to identify which anion is present in each solution, carry out a further test. Record your test and observations. [1] (iii) Identify which solution contains which ion. The CO32– ion is present in …… . The Cl – ion is present in …… . The Br – ion is present in …… . [1] [Total: 13] Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+ 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 hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint 4 Tests for elements element test and test result iodine, I2 gives blue-black colour on addition of starch solution Important values, constants and standards molar gas constant R = 8.31 J K–1 mol–1 Faraday constant F = 9.65 × 104 C mol–1 Avogadro constant L = 6.022 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 at s.t.p. (101 kPa and 273 K) Vm = 24.0 dm3 mol–1 at room conditions ionic product of water Kw = 1.00 × 10–14 mol2 dm–6 (at 298 K (25 °C)) specific heat capacity of water c = 4.18 kJ kg–1 K–1 (4.18 J g–1 K–1) 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 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson – – – – – – – – – – – – – – – – – 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. 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