Chemistry 2023 · May/June

Three students are asked to prepare samples of benzoic acid, C COOH, from the alkaline hydrolysis of methyl benzoate, C COOCH . Sodium benzoate, C COONa, is the salt formed, which is then acidified with hydrochloric acid to form C COOH. COOCH

+ NaOH(aq) COONa(aq) + CH OH

COONa(aq) + HCl (aq) COOH

+ NaCl (aq) The students use the following method. step 1 Put 1.242 g of liquid methyl benzoate into a 50 cm round-bottomed flask. step 2 Prepare 100.0 cm of 1.00 mol dm aqueous sodium hydroxide, NaOH(aq). step 3 Add 10 cm , an excess, of the prepared NaOH(aq) to the round-bottomed flask. step 4 Add some anti-bumping granules to the round-bottomed flask. step 5 Fit a condenser to make reflux apparatus. Reflux the reaction mixture for 20 minutes. step 6 Allow the reaction mixture to cool and carefully pour the liquid into a beaker. step 7 Acidify the liquid with dilute hydrochloric acid. step 8 Filter the mixture formed in step 7. step 9 Purify the benzoic acid by recrystallisation from hot water. Filter, dry and record the mass of pure benzoic acid obtained.

(i) Calculate the volume of methyl benzoate used in step 1. Give your answer to the nearest 0.05 cm [density: methyl benzoate, 1.08 g cm volume of methyl benzoate = cm

Identify a suitable piece of apparatus to measure the volume of methyl benzoate required in step 1.

Calculate the mass of NaOH

that is needed to prepare the solution in step 2. mass of NaOH

Student 1 added the mass of NaOH

calculated in

Student 2 prepared 0.100 mol dm NaOH(aq) instead of 1.00 mol dm NaOH(aq) in step 2. State how this would affect the final mass of benzoic acid formed. Explain, using calculations, how you came to this conclusion. : methyl benzoate, 136; benzoic acid, 122] effect on mass explanation

(i) Explain why it is necessary to reflux the mixture in step 5.

Explain why a naked flame is not used in step 5.

Explain the purpose of transferring the liquid in step 6.

(i) Explain what the students should do to confirm that the mixture has been acidified in step 7.

Describe what you would expect to observe as the sodium benzoate mixture is acidified in step 7.

Suggest why it is necessary to cool the mixture before filtering in step 9.

Pure benzoic acid has a melting point of 122 °C. The product made by student 1 has a melting point of 119 °C. This student suggests the melting point of the product was lower than expected because it contained some water. Explain what the student should do to ensure that the product no longer contains water.

(i) Calculate the maximum mass of benzoic acid that can be formed from 1.242 g of methyl benzoate. : methyl benzoate, 136; benzoic acid, 122] maximum mass of benzoic acid = g

Student 3 produces 0.825 g of benzoic acid from 1.242 g of methyl benzoate. Calculate the percentage yield of benzoic acid produced by student 3. percentage yield = [1] [Total: 17]

Pure water dissociates according to the equation shown. O(l)

The equilibrium constant for this reaction is known as K , the ionic product of water. = [H

] [OH

] At 25 °C, K has a value of 1.00 × 10-14mol . The pH of pure water at 25 °C is 7.00. A student finds the pH of pure water at 35 °C is 6.84. The student investigates how temperature affects the pH of pure water and determines a value for the enthalpy change for the dissociation of water, ΔH. The student records the pH of pure water at different temperatures. A line of best fit from the results of the student's investigation is shown in Fig. 2.1. 6.30 6.40 6.50 6.60 6.70 6.80 6.90 7.00 7.10 7.20 20 25 30 35 40 45 50 55 temperature / °C

(i) Use Fig. 2.1 to calculate the hydrogen ion concentration of pure water at 45 °C. pH = -log [H

] hydrogen ion concentration = mol dm

Calculate the value of K for pure water at 45 °C. = mol

State the relationship between K and temperature.

The student determines K at a range of temperatures. The results are shown in Table 2.1. Table 2.1 temperature, T/ K/ K / mol log K 283 2.93 × 10-15 293 6.81 × 10-15 303 1.47 × 10-14 313 2.92 × 10-14 323 5.48 × 10-14 333 1.09 × 10-13 343 1.45 × 10-13

Complete Table 2.1. Record to three significant figures using standard form. Record log K to two decimal places.

Plot a graph on the grid to show the relationship between log Kand . Use a cross (×) to plot each data point. Draw a line of best fit.

Circle the one point on the graph that you consider to be most anomalous.

Suggest one reason to explain the anomalous point you have circled. Assume there was no error in determining K

Use your graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. These must be selected from your line of best fit. coordinates 1 coordinates 2 gradient = [2] 2.8 -14.6 -14.4 -14.2 -14.0 -13.8 -13.6 -13.4 -13.2 -13.0 -12.8 -12.6 -12.4-12.2 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 log K × 10/ K

The relationship between log Kand is given by the equation shown. log K-ΔH 2.303 RT + constant Use the gradient determined in

, use the value -2550. This is not the correct value. Δ H = kJ mol [2] [Total: 13] Important values, constants and standards molar gas constant R = 8.31 J Kmol Faraday constant F = 9.65 × 10 C mol Avogadro constant L = 6.022 × 10mol electronic charge e = -1.60 × 10-19 molar volume of gas V = 22.4 dmmol at s.t.p. (101 kPa and 273 K) = 24.0 dmmol at room conditions ionic product of water K = 1.00 × 10-14mol (at 298 K (25 °C)) specific heat capacity of waterc = 4.18 kJ kg (4.18 J g Group The Periodic ble of Elements hydroge 1.0 He helium 4.0 34567893Li lithium 6.9 beryllium 9.0 atomic number atomic symbolKey name relative atomic mas sodiu 23.0 magnesium 24.319K potassium 39.1 calcium 40.1 rubidiu 85.5 38Sr strontium 87.6 caesiu 132.9 barium 137.3 87Fr francium Ra radium boron 10.813A aluminiu 27.0 galliu 69.749I indium 1 14.8 81T thallium 204.4 carbon 12.0 14Si silicon 28.1 germanium 72.6tin 18.7 82 lead 207.2 22Ti titanium 47.9 40Zr zirconium 91.2 72Hf hafniu 178.5 104 Rf rutherfordiu23V vanadium 50.9 niobiu 92.9 73Ta tantalum 180.9 105 Db dubnium 24Cr chromium 52.0 molybdenum 95.974W tungsten 183.8 106 Sg seaborgium manganes 54.9 43Tc technetium rheniu 186.2 107 Bh bohriu 26Fe iron 55.8 ruthenium 101.1 osmium 190.2 108 Hs hassium Co cobalt 58.9 rhodiu 102.977I iridiu 192.2 109 Mt meitnerium 28Ni nickel 58.7 palladiu 106.4 78Pt platinum 195.1 darmstadtium copper 63.5 Ag silver 107.9 gold 197.0 111 Rg roentgeniu zinc 65.4 cadmiu 12.4 80 mercury 200.6 coperniciu fleroviu livermorium nitrogen 14.015P phosphorus 31.0 33As arseni 74.9 antimony 121.8 83Bi bismuth 209.0 oxygen 16.016S sulfur 32.1 seleniu 79.0 52Te tellurium 127.6 polonium fluorine 19.017C chlorin 35.5 35Br bromin 79.953I iodine 126.9 85At Ne neon 20.2 18Ar argon 39.9 36Kr krypton 83.8 Xe xenon 131.3 Rn rado nihonium moscoviu tennessine oganesso 21Sc scandium 45.039Y yttrium 88.9 57-71 lanthanoids 89-103 actinoids 57La lanthanum 138.9 89Ac lanthanoids actinoids actinium Ce cerium 140.1 90Th thorium 232.0 59Pr praseodymium 140.9 protactinium 231.0 neodymium 144.492U uraniu 238.0 promethium neptuniu samarium 150.4 plutonium europium 152.0 americiu gadoliniu 157.3 Cm curium 65Tb terbium 158.9 97Bk berkeliu dysprosiu 162.5 98Cf californium holmiu 164.9 99Es einsteinium 68Er erbium 167.3 100 Fm fermium Tm thuliu 168.9 101 Md mendeleviu ytterbium 173.1 102 No nobeliu 71Lu lutetium 175.0 103 Lr lawrencium To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series.