Chemistry 9701/52 February/March 2024: question paper and mark scheme

Question

Sea water contains about 20 g dm–3 of chloride ions, Cl –(aq). The exact concentration of Cl –(aq) in sea water can be determined by titration with aqueous silver ions, Ag+(aq), using aqueous potassium chromate(VI), K2CrO4(aq), as an indicator. When aqueous silver nitrate, AgNO3(aq), is added to a sample of sea water, silver ions react with chloride ions to form a precipitate of silver chloride. Ag+(aq) + Cl –(aq) AgCl (s) When all of the Cl –(aq) has reacted with Ag+(aq), the presence of unreacted Ag+(aq) is detected by chromate(VI) ions, CrO42–(aq). A red precipitate of Ag2CrO4(s) is seen. 2Ag+(aq) + CrO42–(aq) Ag2CrO4(s) The amount of Ag+(aq) reacting with Cl –(aq) in the sample of sea water can be calculated in order to determine the concentration of Cl –(aq) in the sample of sea water. A student uses the following method. step 1 Use a weighing boat to weigh by difference approximately 10.6 g of AgNO3(s) into a 100 cm3 glass beaker. step 2 Use the sample of AgNO3(s) in the glass beaker to prepare 250.0 cm3 of AgNO3(aq). step 3 Transfer this solution into a dark brown glass bottle. Label this solution X. step 4 Collect a sample of sea water and remove any solid material present. step 5 Transfer 10.00 cm3 of the sea water into a conical flask. step 6 Add 1 cm3 of K2CrO4(aq) to the conical flask. step 7 Rinse a burette in preparation for the titration. step 8 Fill the burette with solution X. step 9 Slowly add solution X to the conical flask until the white precipitate turns red. This is the end-point. (a) Describe how the student should carry out step 1. Include a table in your answer to show how this process is recorded. [2] (b) Describe how the student should prepare 250.0 cm3 of AgNO3(aq) in step 2, starting with the AgNO3(s) in the 100 cm3 beaker in step 1. …… [3] (c) Suggest why solution X is kept in a dark brown glass bottle in step 3 rather than a colourless glass bottle. …… [1] (d) Suggest how solid material should be removed from sea water in step 4. …… [1] (e) Identify the most appropriate piece of equipment that you would use to: (i) transfer 10.00 cm3 of sea water from the dark brown bottle to a conical flask in step 5 …… [1] (ii) add 1 cm3 of K2CrO4(aq) to the conical flask in step 6. …… [1] (f) Chromate(VI) solutions are known to be carcinogenic. State what precaution should be taken when using K2CrO4(aq) in step 6 other than wearing safety goggles. …… [1] (g) State what the burette should be rinsed with in step 7. …… [1] (h) The student obtains the results shown in Table 1.1. Table 1.1 rough titration titration 1 titration 2 titration 3 final volume / cm3 23.40 45.75 22.60 45.05 initial volume / cm3 0.00 23.40 0.00 22.60 titre / cm3 (i) Complete Table 1.1. [1] (ii) Calculate the mean titre to be used in the calculations. Show your working. mean titre = …… cm3 [1] (iii) Use the mean titre from (h)(ii) to calculate the concentration of chloride ions in the sample of sea water. Assume the mass of solid silver nitrate used in step 2 was 10.62 g. concentration = …… mol dm–3 [3] (iv) Calculate the percentage error in the titre in titration 2. Show your working. percentage error = …… % [1] (i) Spectroscopic analysis of the sample of sea water accurately determined the concentration of Cl –(aq) to be lower than that determined by titration with Ag+(aq). Suggest why the student’s method gave a higher value. …… [1] [Total: 18]

Question

A student wants to investigate the rate of the hydrolysis of methyl methanoate, HCOOCH3. HCOOCH3 + H2O HCOOH + CH3OH The reaction is catalysed by dilute hydrochloric acid, HCl (aq). The amount of methanoic acid, HCOOH, produced as the reaction progresses can be monitored by titration with aqueous sodium hydroxide, NaOH(aq), of known concentration using thymolphthalein as the indicator. To determine this, the volume of NaOH(aq) needed to neutralise the H+(aq) from the catalyst needs to be found beforehand. The student uses the following procedure. step 1 Add approximately 150 cm3 of iced water to a 250 cm3 conical flask, A. step 2 Add 200 cm3 of 0.250 mol dm–3 HCl (aq) to a 500 cm3 conical flask, B. Conical flask B is the flask in which the reaction takes place. step 3 Transfer 2.00 cm3 of 0.250 mol dm–3 HCl (aq) from conical flask B to conical flask A. Carry out a single titration of the contents of conical flask A with NaOH(aq) of known concentration. step 4 Add 10.0 cm3 of methyl methanoate to conical flask B, swirl the reaction mixture and immediately start a stopwatch. step 5 After 1 minute transfer 2.00 cm3 of the reaction mixture from conical flask B into conical flask A. Carry out a further single titration of the contents of conical flask A against NaOH(aq). Do not empty the contents of conical flask A between titrations. step 6 After 10 minutes transfer 2.00 cm3 of the reaction mixture from conical flask B into conical flask A. Titrate the contents of conical flask A against NaOH(aq). step 7 Repeat step 6 at intervals of 10 minutes for 1 hour. (a) State which step is used to determine the concentration of H+(aq) ions from the catalyst in the mixture. …… [1] (b) The iced water in conical flask A is used to significantly reduce the rate of reaction. Suggest two reasons why the rate of reaction is significantly reduced when the reaction mixture is transferred to conical flask A. reason 1 …… reason 2 …… [2] (c) Table 2.1 shows the readings taken by the student. The titrations in steps 4–7 show the volume of NaOH(aq) needed to neutralise both the H+(aq) ions from the catalyst, HCl (aq), and from the HCOOH produced in the reaction. volume of NaOH(aq) needed, in cm3, to neutralise H+(aq) from catalyst = 11.40 cm3 volume of NaOH(aq), in cm3, used to neutralise H+(aq) from HCOOH at time, t = Vt volume of NaOH(aq), in cm3, used to neutralise H+(aq) from HCOOH at 60 min = V∞ Table 2.1 total volume of NaOH(aq) needed time, t Vt (V∞ – Vt) reading to neutralise total amount of H+(aq) / min / cm3 / cm3 / cm3 1 1 12.60 2 13 17.70 3 20 19.90 4 30 22.10 5 40 6 50 24.90 7 60 25.90 The student forgot to take reading 5. (i) Complete Table 2.1. [2] (ii) Identify the independent variable. …… [1] (iii) Identify one variable that needs to be controlled, apart from concentrations and volumes of solutions. …… [1] (iv) Reading 2 should have been taken at 10 minutes and not at 13 minutes. State whether this result should have been included or not. Explain your answer. …… [1] (v) Plot a graph on the grid in Fig. 2.1 to show the relationship between (V∞ – Vt) and time. Use a cross (×) to plot each data point. Draw a line of best fit. 14 12 10 8 (V∞ – Vt) / cm3 6 4 2 0 0 10 20 30 40 50 60 time, t / min Fig. 2.1 [2] (vi) Reading 5 was not taken. Use the graph to predict the total volume of NaOH(aq) needed to neutralise the total amount of H+(aq) at 40 minutes. volume of NaOH(aq) = …… [1] (vii) It is not possible to repeat the experiment. State whether the data from the experiment is reliable. Justify your answer. …… [1] [Total: 12] 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. 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