Chemistry 9701/36 October/November 2025: question paper and mark scheme

Question

Many hydrated salts decompose when heated, losing water of crystallisation. The number of molecules of water of crystallisation, x, in hydrated aluminium sulfate can be determined by heating until it becomes anhydrous: x is an integer. Al 2(SO4)3•xH2O(s) Al 2(SO4)3(s) + xH2O(g) FB 1 is hydrated aluminium sulfate, Al 2(SO4)3•xH2O. (a) Method • Weigh the crucible with its lid. Record the mass. • Add between 1.80 and 2.00 g of FB 1 to the crucible. • Weigh the crucible, lid and FB 1. Record the mass. • Place the crucible on the pipeclay triangle. Gently heat the crucible and contents for approximately 2 minutes with the lid on. • Remove the lid. Heat the crucible and contents strongly for approximately 5 minutes. • Replace the lid and leave the crucible and residue to cool for at least 5 minutes. While the crucible is cooling, you should begin work on Questions 2 or 3. • Reweigh the crucible and contents with the lid on. Record the mass. • Remove the lid. Heat the crucible and contents strongly for a further 2 minutes. • Replace the lid and leave the crucible and residue to cool for at least 5 minutes. • Reweigh the crucible and residue with the lid on. Record the mass. • Calculate and record the mass of FB 1 used, the mass of residue obtained and the mass lost during heating. Results I II III IV V [5] * 0000800000003 * DFD 3 (b) Calculations (i) Calculate the amount, in mol, of water of crystallisation lost during the thermal decomposition of FB 1. amount of H2O lost = …… mol [1] (ii) Calculate the amount, in mol, of anhydrous residue produced by the thermal decomposition. Show your working. amount of Al 2(SO4)3 produced = …… mol [1] (iii) Calculate the number of molecules of water of crystallisation in the formula of hydrated aluminium sulfate, Al 2(SO4)3•xH2O. x = …… [1] (c) (i) State how the appearance of the residue compares with the appearance of the hydrated solid before heating. …… [1] (ii) Suggest why the crucible and contents are heated with the crucible lid on for the first two minutes of the experiment. …… [1] (iii) A student carries out the experiment in (a), but obtains a value for x that is higher than expected. The student suggests that this could be because the hydrated aluminium sulfate is contaminated with some anhydrous aluminium sulfate. State whether the student’s suggestion is correct. Explain your answer. …… [1] [Total: 11] * 0000800000004 * DFD 4

Question

The number of molecules of water of crystallisation, y, in hydrated iron(II) sulfate can be determined by titration with acidified potassium manganate(VII): y is an integer. FB 2 is aqueous iron(II) sulfate, containing 30.00 g dm–3 of FeSO4•yH2O. FB 3 is aqueous potassium manganate(VII), containing 3.48 g dm–3 of KMnO4. FB 4 is 1.0 mol dm–3 sulfuric acid, H2SO4. (a) Method • Fill the burette with FB 3. • Pipette 25.0 cm3 of FB 2 into a conical flask. • Use the 25 cm3 measuring cylinder to transfer approximately 10 cm3 of FB 4 to 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. • Record, in a suitable form below, all your burette readings and the volume of FB 3 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 be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 2 required …… cm3 of FB 3. [1] * 0000800000005 * DFD 5 (c) Calculations (i) Calculate the amount, in mol, of potassium manganate(VII) present in the volume of FB 3 in (b). Show your working. amount of KMnO4 = …… mol [2] (ii) An incomplete equation for the reaction of iron(II) ions with manganate(VII) ions is shown. The mole ratio of Fe2+ and MnO4– is given correctly. Complete the equation. 5Fe2+(aq) + MnO4–(aq) + …… H+(aq) …… Fe3+(aq) + Mn2+(aq) + ……H2O(l) [1] (iii) Calculate the concentration of iron(II) sulfate, in mol dm–3, in FB 2. concentration of FeSO4 = …… mol dm–3 [1] (iv) Calculate the value of y in FeSO4•yH2O. y = …… [2] (d) A student suggests that the experiment is more accurate if FB 4 is measured with a pipette. State whether you agree with the student. Explain your answer. …… [1] (e) Aqueous solutions of iron(II) sulfate are slowly oxidised by air. State what effect this oxidation would have on the value of y calculated in (c)(iv). Explain your answer. …… [1] [Total: 16] * 0000800000006 * DFD 6 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. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Question

(a) (i) Use very small quantities of solid FB 5 and carry out each of the tests described in Table 3.1. Identify any gases produced. Table 3.1 test observations Test 1 Pour a 1 cm depth of aqueous iron(III) chloride into a test-tube. Add a small spatula measure of FB 5. Leave the test-tube to stand for about 3 minutes, then pour off some of the solution into another test-tube and add aqueous sodium hydroxide. Test 2 Pour a 1 cm depth of aqueous copper(II) sulfate into a test-tube. Add a small quantity of FB 5. Leave the test-tube to stand for about 3 minutes. Test 3 Pour a 1 cm depth of dilute sulfuric acid into a test-tube and add 2 drops of aqueous copper(II) sulfate. Then add a small quantity of FB 5. [4] * 0000800000007 * DFD 7 (ii) FB 6 is the filtrate obtained after filtering the mixture that remains at the end of Test 3 in (a)(i). Add aqueous ammonia to FB 6. Record your observations. …… [1] (iii) Identify FB 5. FB 5 is …… . [1] (iv) Using your observations, explain why the reaction in Test 2 is a redox reaction. …… [1] (v) Give the ionic equation for the first reaction observed in (a)(ii). Include state symbols. …… [1] (b) FB 7 contains one anion and one cation. The anion contains oxygen but not nitrogen. Both ions are listed in the Qualitative analysis notes. (i) Transfer a small spatula measure of FB 7 into a hard-glass test-tube. Heat gently at the start, then strongly until no further change occurs. Leave the test-tube to cool. Record all your observations. Identify any gases produced. …… [3] (ii) Carry out one further positive test to confirm the identity of the anion in FB 7. Record only the results shown in a positive test. Describe the test you carry out and the observations you make in the space below. The anion in FB 7 is …… . [2] [Total: 13] DFD 8 DFD 9 * 0000800000010 * DFD 10 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+ * 0000800000011 * DFD 11 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 = 6.02 × 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 DFD 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 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 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 138.9 140.1 140.9 144.2 – 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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