Chemistry 9701/36 October/November 2024: question paper and mark scheme

Question

Sodium carbonate reacts with hydrochloric acid to release carbon dioxide as shown. Na2CO3(s) + 2HCl (aq) 2NaCl (aq) + CO2(g) + H2O(l) You will find the percentage purity in a sample of impure sodium carbonate by reacting it with excess hydrochloric acid and measuring the volume of carbon dioxide formed. You may assume that the impurity does not react with acid to produce a gas. FB 1 is impure sodium carbonate, Na2CO3. FB 2 is hydrochloric acid, HCl. (a) Method ● Weigh the container with FB 1. Record the mass. ● Fill the tub with water to a depth of approximately 5 cm. ● Fill the 250 cm3 measuring cylinder completely with water. Holding a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. ● Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub. ● Use the 50 cm3 measuring cylinder to transfer 50.0 cm3 of FB 2 into the flask labelled X. Check the bung fits tightly into the neck of flask X, clamp flask X and place the delivery tube into the inverted 250 cm3 measuring cylinder. ● Remove the bung from the neck of the flask. Tip all the FB 1 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. ● Replace the flask in the clamp and leave until the fizzing has stopped. ● Remove the flask from the clamp occasionally, swirl it and replace the flask in the clamp. ● Weigh the empty container that held FB 1. Record the mass. ● Calculate and record the mass of FB 1 added. ● When no more gas is collected, record the final volume of gas produced. You may wish to start Question 2 while the gas is being produced. Results I II III IV [4] (b) Calculations (i) Give your answers to each part of (b)(ii), (b)(iii) and (b)(iv) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of carbon dioxide collected in the 250 cm3 measuring cylinder. amount of CO2 = …… mol [1] (iii) Use your answer to (b)(ii) to deduce the amount, in mol, of the sodium carbonate present in the FB 1 you used in your experiment. amount of Na2CO3 = …… mol Use your answer to calculate the mass, in g, of sodium carbonate in your sample of FB 1. mass of Na2CO3 = …… g [1] (iv) Calculate the percentage purity of FB 1. percentage purity = …… % [1] (c) Even though the bung was replaced quickly, some carbon dioxide was lost. Suggest a change you could make to minimise gas loss at this stage. …… [1] (d) Some carbon dioxide is not collected because it is slightly soluble in water. State a change you could make to reduce the solubility of the gas. Do not suggest using a liquid other than water in your tub or changing the volume of water used. …… [1] (e) State the uncertainty in a single reading of your balance. uncertainty = ± …… g Calculate the maximum percentage error in the mass of FB 1 used in (a). maximum percentage error = …… % [1] [Total: 11]

Question

Many metal carbonates, such as magnesium carbonate, decompose to form the metal oxide when heated. MgCO3(s) MgO(s) + CO2(g) Other metal carbonates, such as sodium carbonate, Na2CO3, do not decompose at the temperature produced by a Bunsen burner. FB 3 is a mixture that contains only sodium carbonate, Na2CO3, and magnesium carbonate. You will carry out an experiment involving thermal decomposition to find the percentage of each of these metal carbonates in this mixture. (a) Method ● Weigh the empty crucible with its lid. Record the mass. ● Transfer all the FB 3 from the container into the crucible. ● Weigh the crucible, lid and FB 3. Record the mass. ● Calculate and record the mass of FB 3 used. ● Place the crucible and contents on the pipe-clay triangle. ● Heat the crucible gently, with the lid on, for approximately 1 minute. ● Heat strongly, with the lid off, for a further 5 minutes. ● Leave the crucible with its contents until it is cool. While the crucible is cooling, you may wish to begin work on Question 3. ● When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. ● Heat the crucible strongly, with the lid off, for approximately 4 minutes. ● Allow the crucible and contents to cool. ● When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass of residue. ● Calculate and record the mass of carbon dioxide produced. Leave the crucible and contents to become completely cool for use in Question 2(c). Results I II III IV V [5] (b) Calculations MgCO3(s) MgO(s) + CO2(g) (i) Calculate the amount, in mol, of carbon dioxide produced in the decomposition. amount of CO2 = …… mol [1] (ii) Calculate the mass of magnesium carbonate in FB 3. mass of MgCO3 = …… g [1] (iii) Calculate the percentages by mass of magnesium carbonate and sodium carbonate in FB 3. percentage by mass of MgCO3 = …… % percentage by mass of Na2CO3 = …… % [1] (c) (i) Add a few drops of water to the cool residue in the crucible. Use universal indicator to test the pH of the solution formed. Tick (3) one box to show the direction of the temperature change. pH = …… temperature goes up temperature goes down [1] (ii) Use these observations and the information about the thermal decomposition of magnesium carbonate to write an equation for the reaction in (c)(i). Include state symbols and the sign of ΔH. …… [2] (iii) Suggest how you would show that sodium carbonate had not decomposed during the reaction in (a). State the reagent(s) and observations. Do not carry out your test. …… [2] [Total: 13] 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

FB 4 is a mixture of two salts that each contain one cation and one anion. All the ions present are in the Qualitative analysis notes. (a) Place a small spatula measure of FB 4 into a hard-glass test-tube. Heat the tube gently at first and then more strongly. Record all your observations and identify any gas given off. …… [4] (b) To a 5 cm depth of distilled water in a boiling tube, add a spatula measure of FB 4. Shake the tube to dissolve the FB 4. (i) Carry out the following tests using a 1 cm depth of this FB 4 solution in a test-tube for each test. Record your observations in Table 3.1. Table 3.1 test observations Test 1 Add aqueous sodium hydroxide, then transfer the mixture into a boiling tube, add a piece of aluminium foil and heat gently. Test 2 Add an equal volume of dilute nitric acid, then add a few drops of aqueous silver nitrate. Test 3 Add aqueous barium chloride or barium nitrate, then add dilute nitric acid. Test 4 Add aqueous sodium carbonate dropwise with shaking until in excess. [5] (ii) Use your observations in Table 3.1 to identify two anions which must be present in FB 4. …… [2] (iii) Carry out further tests to confirm or identify which two cations are present in FB 4. Record the reagents and conditions needed, your observations and your conclusions in a suitable table. [5] [Total: 16] 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 3 4 5 6 7 8 9 10 11 12 23.0 24.3 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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