Chemistry 9701/33 February/March 2025: question paper and mark scheme

Question

Potassium alum is a hydrated salt containing aluminium ions, potassium ions and sulfate ions. 1 mol of hydrated potassium alum contains 12 mol of water of crystallisation. Hydrated potassium alum decomposes when heated, losing its water of crystallisation and becoming anhydrous. You will determine the formula of potassium alum by heating the hydrated salt until it becomes anhydrous. FA 1 is hydrated potassium alum. (a) Method • Weigh a crucible with its lid. Record the mass in the space for results. • Add all of the FA 1 to the crucible. • Weigh the crucible and lid with FA 1. Record the mass. • Calculate and record the mass of FA 1 added. • Place the crucible on the pipe-clay triangle. Heat the crucible and contents gently 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 may begin work on Question 2 or Question 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 residue obtained. Results I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of water of crystallisation lost during the thermal decomposition of FA 1. amount of H2O lost = …… mol [1] (ii) Use the information given and your answer to (b)(i) to determine the amount, in mol, of potassium alum used. amount of potassium alum = …… mol [1] (iii) Calculate the relative formula mass, Mr, of anhydrous potassium alum. Mr = …… [1] (iv) Anhydrous potassium alum contains aluminium ions, potassium ions and sulfate ions. 1 mol of potassium alum also contains 1 mol of aluminium ions. Use the Mr you have calculated in (b)(iii) to suggest the formula of anhydrous potassium alum. Show your working. The formula = …… . [1] (c) (i) The uncertainty in a single balance reading for a two decimal place balance is 0.01 g. Calculate the maximum percentage error in your measurement of the mass of the residue of anhydrous potassium alum. Show your working. maximum percentage error = ……% [1] (ii) A student obtains a higher value for the relative formula mass, Mr, of anhydrous potassium alum than expected. The student incorrectly suggests that this is because some of the anhydrous potassium alum residue decomposes to aluminium oxide and potassium oxide during strong heating. Explain why the student’s suggestion is not correct. …… [1] [Total: 11]

Question

Many oxidising agents are able to oxidise acidified potassium iodide to iodine in acidic conditions. The amount of iodine produced can be determined by titrating it with aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) 2NaI(aq) + Na2S4O6(aq) You will determine the change in oxidation state of an oxidising agent when it reacts with iodide ions. FA 2 is aqueous sodium thiosulfate, containing 22.00 g Na2S2O3•5H2O (Mr = 248.2) in 1.00 dm3. FA 3 is a 0.0175 mol dm–3 solution of an oxidising agent. FA 4 is 0.50 mol dm–3 potassium iodide, KI. FA 5 is 1.00 mol dm–3 sulfuric acid, H2SO4. FA 6 is starch solution. (a) Method • Fill the burette with FA 2. • Pipette 25.0 cm3 of FA 3 into a conical flask. • Use the 10.0 cm3 measuring cylinder to add 10 cm3 of FA 4, an excess, to the conical flask. • Use the 25.0 cm3 measuring cylinder to add 20 cm3 of FA 5, an excess, to the conical flask. • Add FA 2 from the burette until the solution becomes yellow. • Add about 10 drops of FA 6 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 in the space below, all your burette readings and the volume of FA 2 added in each accurate titration. Keep FA 3, FA 4, FA 5 and FA 6 for use in Question 3. 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 obtain the mean value. 25.0 cm3 of FA 3 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 amount, in mol, of sodium thiosulfate in the volume of FA 2 in (b). amount of Na2S2O3 = …… mol [1] (iii) Calculate the amount, in mol, of iodine that reacts with the amount of sodium thiosulfate in (c)(ii). amount of I2 = …… mol [1] (iv) Calculate the amount, in mol, of FA 3 used to produce the amount of iodine in (c)(iii). amount of FA 3 = …… mol [1] (v) Calculate the amount, in mol, of iodine produced by the reaction of 1 mol of FA 3 with potassium iodide. Give your answer to one decimal place. amount of I2 = …… mol [1] (vi) The oxidising agent in FA 3 is a compound of a transition metal, M. The redox reaction of FA 3 with iodide ions produces M2+ ions. Use your answer to (c)(v) to calculate the change in the oxidation state of M during this reaction. Show your working. The oxidation state of M changes from …… to …… . [2] (d) A student suggests that the experiment in (a) would be more accurate if the FA 5, sulfuric acid, is measured using a pipette. State whether the student is correct. Explain your answer. …… [1] [Total: 16] 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) Carry out the following tests using FA 3 and record your observations in Table 3.1. Use a 1 cm depth of FA 3 in a test-tube for each test. Table 3.1 test observations Test 1 Add an equal volume of aqueous sodium hydroxide, then add an equal volume of aqueous sodium sulfite. Shake the mixture in the tube, then add sulfuric acid, FA 5. Test 2 Add an equal volume of sulfuric acid, FA 5, then add a small spatula measure of zinc. Allow the mixture to stand. Test 3 Add an equal volume of aqueous hydrogen peroxide. Give the formula of the gas formed in Test 3. The gas formed is …… . [4] (b) (i) FA 7 is a solution containing four ions, three of which are listed in the Qualitative analysis notes. Carry out the following tests and record your observations in Table 3.2. Use a 1 cm depth of FA 7 for each test. A boiling tube must be used for Test 1 and a test-tube for the other tests. Table 3.2 test observations Test 1 Add 0.5 cm depth of aqueous sodium hydroxide to a boiling tube, then warm the mixture carefully, then add aluminium foil. Test 2 Add several drops of aqueous potassium iodide, FA 4, then add a few drops of starch solution, FA 6. Test 3 Add the pieces of magnesium. Test 4 Add a few drops of aqueous barium nitrate or aqueous barium chloride, then add hydrochloric acid. Test 5 Add a few drops of aqueous silver nitrate, then add aqueous ammonia. [6] (ii) Give the formula of each of the four ions in FA 7. The ions are …… , …… , …… and …… . [2] (iii) Give the ionic equation for one reaction that takes place in Test 1 or Test 3 of (b)(i). Include state symbols. …… [1] [Total: 13] Qualitative analysis notes 1 Reactions of aqueous 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 F = 9.65 × 104 C mol–1 Faraday constant 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 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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