Cambridge A Level Chemistry 9701 — 2022 Feb/March Paper 5 · Variant 2

1 A student has a sample of copper(II) sulfate crystals, CuSO4•xH2O. The student wants to show that the value of x is 5. The student uses the following method. step 1 Weigh a clean crucible on a balance reading to two decimal places. Record the mass. step 2 Place the sample of CuSO4•xH2O into the crucible. Record the mass. step 3 Heat the crucible gently for about 1 minute then strongly for about 4 minutes. step 4 Weigh the crucible and contents. Record the mass. (a) Identify the instruction that is missing between step 3 and step 4. .............................................................................................................................................. [1] (b) Explain why gentle heating takes place in step 3. .............................................................................................................................................. [1] (c) Name the apparatus that should be used to hold the crucible during heating. .............................................................................................................................................. [1] (d) The method is incomplete. State the step(s) that should be carried out to complete the method. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (e) The student records their results in Table 1.1. Table 1.1 mass / g mass of crucible 13.60 mass of crucible + contents before heating 21.09 mass of crucible + contents at the end of experiment 17.94 (i) Calculate the experimental value of x from these results. experimental value of x = .............................. [3] (ii) Suggest why the experimental value of x varies from the expected value of 5. If you were unable to obtain an answer to (e)(i), use the experimental value x = 6.9. This is not the correct answer. ............................................................................................................................................. ....................................................................................................................................... [1] (f) The empty crucible weighs 13.60 g. Calculate the percentage error in this measurement. Show your working. percentage error = .............................. [1] [Total: 9]

Mark scheme: 1(a) allow (crucible / contents) to cool 1 1(b) to prevent spitting of crystals 1 1(c) pipe-clay triangle 1 1(d) heat to constant mass 1 1(e)(i) M1: (mass H2O) = 3.15 (g) AND (mass CuSO4) = 4.34 (g) M2: n(H2O) = 3.15 / 18 = 0.175 AND n(CuSO4) = 4.34 / 159.6 = 0.02719… M3: 0.02719… / 0.02719… AND 0.175 / 0.02719… = 6.44 3 1(e)(ii) If value of x in 1(e)(i) is above 5: CuSO4 must have decomposed / crystals were wet before heating / some crystals were lost when heated (e.g. during spitting) / impurity that decomposes or evaporates If value of x in 1(e)(i) is below 5: did not heat it enough / not all water driven off / unreactive impurity 1 1(f) [(2 × 0.005) / 13.60] × 100 = 0.0735 (%) 1

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2 The conductivity of an ionic solution can be determined by passing an electric current through the solution and measuring the conductivity using a conductivity meter. (a) A student carries out an experiment to measure the conductivity of different solutions of ethanoic acid, CH3COOH, which is a weak acid. The acid dissociation constant, Ka, can be determined from this experiment. The student makes standard solution A, 250.0 cm3 of 2.00 mol dm–3 CH3COOH(aq). (i) State what is meant by a standard solution. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Describe how the student should make standard solution A from pure ethanoic acid. The concentration of standard solution A should be 2.00 mol dm–3 to the nearest three significant figures. Your answer should state the name and capacity of any apparatus that the student should use. A balance is not available. Pure ethanoic acid is a liquid with a density of 1.05 g cm–3 at room temperature. You may wish to write your answer using a series of numbered steps. [Mr: CH3COOH, 60.0] ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [4] (b) The student wears chemically resistant gloves throughout this procedure. Suggest why. .............................................................................................................................................. [1] (c) The student dilutes standard solution A with distilled water to make solutions of different concentrations. The conductivity of these solutions is measured. Table 2.1 shows the results. Table 2.1 1 2 3 4 1 [CH3COOH] conductivity molar conductivity, ΛM / mol dm–3 / S dm–1 / dm2 S mol–1 / dm1.5 mol–0.5 0.0500 3.76 × 10–3 0.0250 2.74 × 10–3 0.0125 1.92 × 10–3 0.00625 1.33 × 10–3 0.003125 1.15 × 10–3 0.0015625 6.68 × 10–4 In order to determine Ka the results must be used to obtain two sets of data. Column 3 is the reciprocal of the square root of the concentration (column 1). Column 4 is the molar conductivity, ΛM, which is found by dividing the conductivity (column 2) by the concentration (column 1). (i) Complete columns 3 and 4 in Table 2.1. Give all values to three significant figures. [2] (ii) Identify the dependent variable in this experiment. ....................................................................................................................................... [1] (iii) The student decided to measure the conductivity of distilled water at the start of the experiment. Suggest why. ............................................................................................................................................. ....................................................................................................................................... [1] (iv) State one variable that should be controlled. ....................................................................................................................................... [1] (d) Plot a graph on the grid to show the relationship between molar conductivity, ΛM, and 1 . Use a cross (×) to plot each data point. Draw a line of best fit that includes the origin. 0.45 0.40 0.35 0.30 0.25 molar conductivity, M / dm2 S mol–1 0.20 0.15 0.10 0.05 0.00 0 5 10 15 20 25 30 1 / dm1.5 mol–0.5 [2] (e) Circle one point on the graph that you consider to be most anomalous. The conductivity meter was correctly functioning. Suggest one reason why this anomaly may have occurred during this experimental procedure. .............................................................................................................................................. [2]

Mark scheme: 2(a)(i) solution of known concentration 1 2(a)(ii) M1: measure 28.55 cm3 of CH3COOH M2: using burette / graduated pipette M3: (transfer/add CH3COOH to) a 250 cm3 volumetric flask M4: top the (volumetric) flask up to (250 cm3) mark with distilled water 4 2(b) (pure) CH3COOH is corrosive 1 2(c)(i) 3 4 4.47 0.0752 6.32 0.110 8.94 0.154 12.6 0.213 17.9 0.368 25.3 0.428 M1: column 3 all values to 3 SF M2: column 4 all values to 3 SF 2 2(c)(ii) conductivity 1 2(c)(iii) to calibrate the conductivity meter 1 2(c)(iv) temperature 1 2(d) M1: 6 points plotted correctly M2: line of best fit drawn 2 Question Answer Marks 2(e) M1: 5th point circled at 17.9, 0.368 M2: ethanoic acid was diluted incorrectly 2 2(f)(i) M1: co-ordinates read and recorded correctly M2: gradient calculated to 3 SF 2 2(f)(ii) M1: (gradient calculated in (f)(i) / 3.91)2 (0.0169 / 3.91)2 = 1.87 × 10–5 M2: mol dm–3 2 2(g) M1: percentage error of experimental value = [(1.34 – 1.28) / 1.34] × 100 = 4.5% M2: valid and the calculated actual percentage error is less than the maximum percentage error 2

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