Chemistry 9701/33 February/March 2023: question paper and mark scheme

Question

Some metal carbonates occur in a basic form which means that the metal hydroxide is also present. The formula of one form of basic zinc carbonate is ZnCO3•2Zn(OH)2•xH2O, where x is an integer. In this experiment you will carry out a thermal decomposition to find the relative formula mass, Mr, and the value of x for a sample of basic zinc carbonate. FA 1 is basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O. (a) Method ● Weigh the empty crucible with its lid. Record the mass. ● Transfer all the FA 1 from the container into the crucible. ● Weigh the crucible, lid and FA 1. Record the mass. ● Place the crucible and contents on a pipe-clay triangle. ● Heat the crucible gently, with the lid on, for approximately 1 minute. ● Heat strongly, with the lid off, for a further 4 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. While the crucible is cooling you may wish to begin work on Question 2 or 3. ● When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. ● Place the crucible and contents on the pipe-clay triangle. Remove the lid. ● Heat the crucible strongly for a further 2 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. ● When the crucible is cool, reweigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass of FA 1 used. ● Calculate and record the mass of residue obtained. Results I II III IV V [5] (b) Calculations The equation for the thermal decomposition is shown. ZnCO3•2Zn(OH)2•xH2O(s) 3ZnO(s) + CO2(g) + (x + 2)H2O(g) (i) Calculate the amount, in mol, of zinc oxide, ZnO, formed after heating. amount of ZnO = …… mol Hence, calculate the amount, in mol, of basic zinc carbonate in your sample of FA 1. amount of ZnCO3•2Zn(OH)2•xH2O = …… mol [1] (ii) Use your answer to (b)(i) and your results in (a) to calculate the relative formula mass, Mr, of basic zinc carbonate, FA 1. Mr of ZnCO3•2Zn(OH)2•xH2O = …… [1] (iii) Use the Periodic Table to calculate the relative formula mass, Mr, of ZnCO3•2Zn(OH)2. Mr of ZnCO3•2Zn(OH)2 = …… [1] (iv) Use your answers to (b)(ii) and (b)(iii) to determine the value of x in ZnCO3•2Zn(OH)2•xH2O. Show your working. x = …… [2] [Total: 10]

Question

In this experiment you will determine the relative formula mass, Mr, of basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O, by an alternative method. A known mass of basic zinc carbonate is reacted with a known volume and concentration of hydrochloric acid, HCl. The acid added is in excess. You will titrate portions of the resulting solution with sodium hydroxide, NaOH, of known concentration. FA 2 has been prepared as follows: 3.52 g of basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O, is reacted with 100 cm3 of 2.00 mol dm−3 hydrochloric acid, HCl. The resulting solution is diluted to 1.00 dm3 with distilled water. FA 3 is 0.150 mol dm−3 sodium hydroxide, NaOH. FA 4 is bromophenol blue indicator. (a) Method ● Fill the burette with FA 3. ● Pipette 25.0 cm3 of FA 2 into a conical flask. ● Add several drops of FA 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 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 FA 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 use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FA 2 required …… cm3 of FA 3. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(vi) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium hydroxide present in the volume of FA 3 in (b). amount of NaOH = …… mol [1] (iii) Use your answer to (c)(ii) to calculate the amount, in mol, of hydrochloric acid present in 1.00 dm3 of FA 2. amount of HCl in 1.00 dm3 of FA 2 = …… mol [1] (iv) Use your answer to (c)(iii) and the information about the preparation of FA 2 to calculate the amount, in mol, of hydrochloric acid that reacted with 3.52 g of basic zinc carbonate. amount of HCl that reacted = …… mol [1] (v) Using the equation given in question 1(b) as a guide, complete the equation for the reaction of hydrochloric acid with basic zinc carbonate. State symbols are not required. ZnCO3•2Zn(OH)2•xH2O + 6HCl …… + …… + …… [1] (vi) Use your answer to (c)(iv) and the equation in (c)(v) to calculate the amount, in mol, of basic zinc carbonate in 3.52 g. amount of ZnCO3•2Zn(OH)2•xH2O = …… mol Hence, calculate the relative formula mass, Mr, of basic zinc carbonate. Mr of ZnCO3•2Zn(OH)2•xH2O = …… [2] (d) A student correctly followed all the instructions in (a) to (c) of this question. The formula mass the student calculated was smaller than the correct value. The student suggests that this may be because the concentration of sodium hydroxide, FA 3, is greater than 0.150 mol dm–3. Explain why the student is correct. …… [2] [Total: 17] 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 5 contains one cation and one anion, both of which are listed in the Qualitative analysis notes. FA 5 is insoluble in water. Transfer approximately half of the sample of FA 5 into a hard-glass test-tube. Heat the test- tube gently at first, then more strongly, until no further change occurs. Record all your observations. …… [2] (b) FA 6 is an aqueous solution containing one cation and one anion. The anion is listed in the Qualitative analysis notes. FA 6 does not contain sulfur. (i) To a 2 cm depth of FA 6 in a test-tube, add a piece of magnesium ribbon. Record all your observations. …… [2] (ii) Write an ionic equation for the reaction between FA 6 and magnesium. Include state symbols. …… [1] (c) (i) Add a small spatula measure of FA 5 to a 2 cm depth of FA 6 in a boiling tube. Record all your observations. The product of this test is FA 7. Label the boiling tube FA 7. …… [1] (ii) To a 1 cm depth of FA 7 in a test-tube, add aqueous ammonia. Record all your observations. …… [1] (d) (i) The same anion is present in both FA 6 and FA 7. The identity of this anion can be confirmed by testing either FA 6 or FA 7 with a pair of reagents. Select two pairs of reagents which may be used to identify the anion present in both FA 6 and FA 7. first pair of reagents: …… and …… second pair of reagents: …… and …… [2] (ii) Use FA 6 to carry out each test using the two pairs of reagents you have selected. Record all your observations in a suitable form below. [2] (e) Use your observations in (a) to (d) to identify the ions present in FA 5 and FA 6. If you are unable to identify an ion write ‘unknown’. cation anion FA 5 FA 6 [2] [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 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