Chemistry 9701/51 October/November 2024: question paper and mark scheme

Question

A student uses the following method to determine the percentage by mass of the painkiller aspirin, C9H8O4(s), in some tablets. step 1 Grind five tablets into a powder. step 2 Use a weighing boat to accurately weigh by difference approximately 0.4 g of powdered tablets into a pear‑shaped flask containing anti‑bumping granules. step 3 Add 25 cm3 of aqueous 1 mol dm–3 sodium hydroxide, NaOH(aq), to the pear‑shaped flask, forming mixture A. step 4 Reflux mixture A for 20 minutes. step 5 Allow mixture A to cool and then filter into a small beaker. Label the filtrate solution B. step 6 Add 30 cm3 of alkaline aqueous iodine to solution B and leave to stand for 1 hour. A precipitate, C, (C6H2I2O)2(s), will form. step 7 Filter the resulting mixture under reduced pressure. Wash the residue, C, with a small volume of cold distilled water. step 8 Allow solid C to dry. step 9 Weigh solid C and record its mass. Alkaline aqueous iodine is irritating to the skin and eyes. (a) Identify an appropriate precaution, other than eye protection and a lab coat, that the student should take when using alkaline aqueous iodine. …… [1] (b) Describe how the student should carry out step 2. Include a results table, with appropriate headings, for the student to fill in. [2] (c) Complete Fig. 1.1 to show how step 4 is carried out in the laboratory. Label your diagram fully. pear-shaped flask mixture A + anti-bumping granules heat Fig. 1.1 [2] (d) (i) The student uses a measuring cylinder to measure the volume of alkaline aqueous iodine in step 6. Suggest why this is a suitable piece of apparatus to use. …… [1] (ii) Suggest why the student leaves the mixture to stand for 1 hour in step 6. …… [1] (iii) Explain why the residue is washed in step 7. …… [1] (iv) Explain why hot distilled water is not used in step 7. …… [1] (e) The equation for the reaction between aspirin, C9H8O4(s), and NaOH(aq), which takes place in step 4, is shown. C9H8O4(s) + 2NaOH(aq) C7H5O3– Na+(aq) + C2H3O2– Na+(aq) + H2O(l) The equation for the reaction in which solid C, (C6H2I2O)2(s), is formed in step 6 is shown. 2C7H5O3– Na+(aq) + 6I2(aq) + 8OH–(aq) (C6H2I2O)2(s) + 8I–(aq) + 2NaHCO3(aq) + 6H2O(l) The student’s results are shown in Table 1.1. Table 1.1 mass of powdered tablets added to the pear‑shaped flask in step 2 0.409 g mass of dry (C6H2I2O)2(s) recorded in step 9 0.764 g (i) Calculate the amount, in mol, of (C6H2I2O)2(s) collected in step 9. [Mr: (C6H2I2O)2, 687.6] amount of (C6H2I2O)2 …… mol [1] (ii) Use your answer to (i) to calculate the mass, in g, of C9H8O4(s) in the powdered tablets added to the flask in step 2. mass of C9H8O4(s) …… g [1] (iii) Use your answer to (ii) to calculate the percentage by mass of aspirin, C9H8O4(s), in the tablets. If you were unable to obtain an answer to (ii) you may use 0.374 g for the mass of C9H8O4(s). This is not the correct value. percentage by mass C9H8O4(s) in the tablets …… [1] (f) Another student follows the same method but does not allow solid C to dry completely in step 8. State and explain the effect that this has on the calculated percentage by mass of aspirin, C9H8O4(s), in the tablets. …… [1] [Total: 13]

Question

Crystal violet, C25H30N3Cl (s), is a purple dye. Some light is absorbed when it passes through C25H30N3Cl (aq). Absorbance is the proportion of light absorbed at a particular wavelength. This is measured using a colorimeter. A graph of absorbance against wavelength for C25H30N3Cl (aq) is shown in Fig. 2.1. absorbance 400 450 500 550 600 650 700 wavelength / nm Fig. 2.1 A student investigates how to determine the concentration of aqueous crystal violet, C25H30N3Cl (aq), using colorimetry. (a) Suggest the best wavelength of light to use in the colorimeter when measuring the concentration of C25H30N3Cl (aq). wavelength = …… nm [1] (b) Solution D is 500.0 cm3 of 2.50 × 10–2 mol dm–3 C25H30N3Cl (aq). (i) Calculate the mass of C25H30N3Cl (s) needed to prepare solution D. Give your answer to three significant figures. [Mr: C25H30N3Cl (s), 407.5] mass of C25H30N3Cl (s) = …… g [1] (ii) The student is given a small beaker containing the mass of C25H30N3Cl (s) calculated in (i). Describe how the student should prepare 500.0 cm3 of solution D. Include the name and capacity of the key apparatus which should be used and describe how the student should ensure the volume is exactly 500.0 cm3. …… [3] (c) A small sample of solution D was diluted to form solution E, 2.50 × 10– 4 mol dm–3 C25H30N3Cl(aq). The student prepares solutions 2 to 6 as shown in Table 2.1. The total volume needed for each of solutions 2 to 6 is 20.00 cm3. Each solution is placed into a colorimeter and the absorbance is measured. (i) Complete Table 2.1 to show the volumes of solution E and distilled water needed to prepare each of the solutions from 2 to 6. Give all volumes to two decimal places. Table 2.1 volume of 2.50 × 10– 4 mol dm–3 volume of [C25H30N3Cl(aq)] solution C25H30N3Cl(aq) distilled water absorbance / cm3 / mol dm–3 (solution E) / cm3 1 0.00 20.00 0.00 0.000 2 0.50 × 10– 4 0.191 3 1.00 × 10– 4 0.270 4 1.50 × 10– 4 0.545 5 2.00 × 10– 4 0.711 6 2.50 × 10– 4 0.860 [1] (ii) Identify the dependent variable. …… [1] (d) (i) Plot a graph of absorbance against [C25H30N3Cl(aq)] on the grid in Fig. 2.2. Use a cross (×) to plot each data point. Draw a straight line of best fit. 1.000 0.900 0.800 0.700 0.600 absorbance 0.500 0.400 0.300 0.200 0.100 0.000 0.00 0.50 1.00 1.50 2.00 2.50 3.00 [C25H30N3Cl (aq)] × 10– 4 / mol dm–3 Fig. 2.2 [2] (ii) Circle the point on the graph you consider to be most anomalous. Suggest one reason why this anomaly may have occurred during this experimental procedure. Assume no error was made in the measurement of absorbance. …… [2] (iii) State the relationship between [C25H30N3Cl(aq)] and absorbance. …… [1] (iv) Suggest how the student could improve the reliability of the data obtained in the experiment in (c). …… [1] Question 2 continues on the next page. (e) The student carries out a further experiment to examine the kinetics of the reaction between crystal violet, C25H30N3Cl (aq), and aqueous sodium hydroxide, NaOH(aq). C25H30N3Cl (aq) + OH–(aq) C25H30N3OH(aq) + Cl –(aq) The disappearance of the purple colour as the reaction proceeds can be monitored by measuring how the absorbance of light by the mixture changes using a colorimeter. The student mixes 5 cm3 of solution 6 with 5 cm3 of NaOH(aq), a large excess, and immediately starts the stopwatch. The resulting mixture is then placed in a colorimeter. The absorbance of this mixture is measured every 100 seconds after starting the stop‑watch. Fig. 2.3 shows a graph of the student’s results. 0.400 0.350 0.300 0.250 absorbance 0.200 0.150 0.100 0.050 0.000 0 100 200 300 400 500 600 700 800 time / s Fig. 2.3 (i) Suggest why it is not possible for the student to measure the absorbance of the mixture at t = 0 s. …… [1] (ii) Use the graph in Fig. 2.3 to find the half‑life, t 12 , starting at 100 s. State the coordinates of both points on the line of best fit used in your calculation. coordinates 1 …… coordinates 2 …… half‑life …… s [2] (iii) Another student repeats the experiment at a different temperature and measures two half‑life values. The values obtained are 420 s and 425 s. Use these values to deduce the order of the reaction with respect to C25H30N3Cl(aq). Explain your answer. order = …… explanation …… …… [1] [Total: 17] 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 16 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. 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