Chemistry 9701/31 October/November 2023: question paper and mark scheme

Question

You will determine the value of x in hydrated sodium carbonate, Na2CO3•xH2O. x is not an integer. You will carry out two methods to determine the value of x. Each method involves sodium carbonate reacting with excess hydrochloric acid to release carbon dioxide. Na2CO3•xH2O(s) + 2HCl (aq) → 2NaCl (aq) + CO2(g) + xH2O(l) (a) Experiment 1 You will measure the volume of carbon dioxide released when hydrated sodium carbonate reacts with excess hydrochloric acid. FA 1 is 0.500 mol dm–3 hydrochloric acid, HCl. FA 2 is hydrated sodium carbonate, Na2CO3•xH2O. Method ● Weigh the container with FA 2. 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. ● Using the 50 cm3 measuring cylinder, transfer 50.0 cm3 of FA 1 into the flask labelled Z. Check that the bung fits tightly into the neck of flask Z, clamp flask Z and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ● Remove the bung from the neck of the flask. Tip all the FA 2 from the container into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. You may need to shake the flask quite vigorously until the gas formed starts to collect in the measuring cylinder. ● Return the flask to the clamp. Leave for several minutes, shaking the flask occasionally. ● Weigh the container with any residual FA 2. Record the mass. ● Calculate the mass of FA 2 added to the flask. Record the mass. ● When no more gas is collected, measure the final volume of gas in the measuring cylinder. Record the volume. Results I II III [3] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide collected in the measuring cylinder at room conditions. amount of CO2 = …… mol Hence deduce the amount, in mol, of sodium carbonate present in the FA 2 you added in your experiment. amount of Na2CO3 = …… mol [1] (ii) Use your answer to (b)(i) and the mass of hydrated sodium carbonate, Na2CO3•xH2O, you used in Experiment 1 to calculate the relative formula mass, Mr, of the Na2CO3•xH2O. Mr of Na2CO3•xH2O = …… [1] (iii) Use your answer to (b)(ii) to calculate the value of x in the Na2CO3•xH2O. Show your working. x = …… [2] (c) A student suggests that it would be better to use hot water in the tub. (i) State whether using hot water would be an improvement. Explain your answer. …… [1] (ii) State the effect, if any, that using hot water would have on the value of x calculated. …… [1] (d) Experiment 2 You will carry out a titration to measure the volume of hydrochloric acid that neutralises an aqueous solution of hydrated sodium carbonate, Na2CO3•xH2O. Na2CO3•xH2O(s) + 2HCl (aq) → 2NaCl (aq) + CO2(g) + xH2O(l) FA 3 is 0.100 mol dm–3 hydrochloric acid, HCl. FA 4 is an aqueous solution containing 14.30 g dm–3 of hydrated sodium carbonate, Na2CO3•xH2O. FA 5 is bromophenol blue indicator. Method ● Fill the burette with FA 3. ● Pipette 25.0 cm3 of FA 4 into a conical flask. ● Add a few drops of FA 5. ● Carry out a rough titration and record your burette readings in the space below. The rough titre is …… cm3. ● Carry out as many titrations as you think necessary to obtain consistent results. ● Make sure your recorded results show the precision of your practical work. ● Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration. I II III IV V VI VII [7] (e) 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 4 required …… cm3 of FA 3. [1] (f) Calculations (i) Give your answers to (f)(ii), (f)(iii) and (f)(iv) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of hydrochloric acid present in the volume of FA 3 you calculated in (e). amount of HCl = …… mol [1] (iii) Use the equation for the neutralisation to deduce the amount, in mol, of sodium carbonate present in 25.0 cm3 of Na2CO3•xH2O. amount of Na2CO3 = …… mol Hence calculate the amount, in mol, of sodium carbonate in 1.00 dm3 of Na2CO3•xH2O. amount of Na2CO3 in 1.00 dm3 = …… mol [1] (iv) Calculate the value of x in the sample of Na2CO3•xH2O. Show your working. x = …… [1] (g) The aqueous solution of Na2CO3•xH2O, FA 4, was prepared by weighing and dissolving the solid to make 1.00 dm3 of solution. Mass of container + Na2CO3•xH2O = 32.509 g Mass of empty container = 18.209 g Mass of Na2CO3•xH2O = 14.300 g (i) State the maximum uncertainty in a single balance reading for the balance used. maximum uncertainty = ± …… g Calculate the maximum percentage uncertainty in this mass of Na2CO3•xH2O. Show your working. maximum percentage uncertainty = ± …… % [1] (ii) Using the method in Experiment 2 a student calculated the relative formula mass, Mr, of Na2CO3•xH2O to be 242.2. Assume that the uncertainty in the mass of FA 4 is the only source of error in the experiment. Calculate the maximum value for the relative formula mass of FA 4. maximum value for the relative formula mass of FA 4 = …… [1] [Total: 23] 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, FA 7 and FA 8 are salts each of which contains nitrogen. Each of the nitrogen-containing ions is different and all are shown in the Qualitative analysis notes. (i) List the nitrogen-containing ions for which you will test. …… and …… and …… . Select reagents to positively identify the nitrogen-containing ions in each salt. Record your tests and the results with each salt in a suitable table in the space below. [8] (ii) Use your observations in (a)(i) to determine the formulae of the nitrogen-containing ions present in FA 6, FA 7 and FA 8. FA 6 …… FA 7 …… FA 8 …… [1] (b) FA 9 and FA 10 contain the same element. You will identify this element by carrying out tests. (i) Test 1 Heat a small spatula measure of FA 9 in a hard-glass test-tube. Heat until the reaction stops. After heating, leave the tube to cool and keep it for Test 2. You may wish to start (b)(ii) while you wait. Record your observations. Name one product of the reaction. product …… Test 2 To the cooled solid product of Test 1, add a 2–3 cm depth of distilled water. Shake the test-tube and then leave the contents to settle. Record your observations. [3] (ii) To a very small spatula measure of FA 9 in a test-tube, add about a 2 cm depth of dilute sulfuric acid and about a 2 cm depth of distilled water. Shake to dissolve the FA 9 and produce FA 9(aq). You will use FA 9(aq) in Test 3 and Test 4. Test 3 To a 1 cm depth of aqueous iron(II) sulfate in a test-tube, add a few drops of FA 9(aq). Record your observations. Test 4 To a 1 cm depth of aqueous potassium iodide in a test-tube, add a few drops of FA 9(aq). Record your observations. [2] (iii) To a small spatula measure of FA 10 in a test-tube, add distilled water to dissolve the FA 10. This solution is FA 10(aq). To a 1 cm depth of FA 10(aq), add aqueous sodium hydroxide. Record your observations. …… [2] (iv) Identify the element that is present in FA 9 and FA 10. element …… [1] [Total: 17] 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

Question

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 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 138.9 140.1 140.9 144.4 – 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

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