Cambridge A Level Chemistry 9701 — 2025 May/June Paper 5 · Variant 4

1 The relative molecular mass, Mr , of an unknown volatile liquid, Y, can be determined experimentally. A student uses the following method to determine the volume of a conical flask. step 1 Weigh a dry conical flask. step 2 Fill the conical flask completely with distilled water. step 3 Weigh the conical flask when filled with distilled water. step 4 Use a thermometer to measure the temperature of the distilled water in the conical flask. The student collects the results shown in Table 1.1. Table 1.1 mass of dry flask / g 31.022 mass of flask filled with water / g 161.175 temperature of water in flask / °C 23.0 laboratory atmospheric pressure / kPa 99.0 (a) Fig. 1.1 shows the variation of the density of water with temperature. 1.000 0.999 0.998 density / g cm–3 0.997 0.996 0.995 16 18 20 22 24 26 28 30 temperature / °C Fig. 1.1 (i) Use Fig. 1.1 to determine the density of water at 23.0 °C. Give your answer to four decimal places. density of water = .............................. g cm–3 [1] (ii) Calculate the volume of the flask, using the density of water determined in (a)(i). volume of flask = .............................. cm3 [2] (b) Another student determines the volume of a different conical flask to be 129.56 cm3. This conical flask is used to determine the Mr of unknown volatile liquid Y. The following method is used. step 1 Cover the opening of the dry conical flask with aluminium foil and secure using a rubber band. step 2 Weigh the conical flask, aluminium foil and rubber band. Record the mass. step 3 Use a syringe with a needle to make a small hole in the aluminium foil and inject approximately 5 cm3 of Y into the conical flask. Remove the syringe. step 4 Place the conical flask in a water-bath containing boiling water. step 5 Allow all of Y to evaporate. Keep the conical flask containing vaporised Y in the boiling water for a further 3 minutes. step 6 Carefully remove the conical flask from the boiling water and dry the outside surface thoroughly. step 7 Weigh the conical flask, aluminium foil, rubber band and contents. Record this mass. (i) Suggest why the conical flask containing vaporised Y is kept in boiling water for 3 minutes in step 5. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Suggest why it is not necessary to determine the mass of liquid Y injected into the conical flask by the syringe in step 3. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) The student thinks that Y is toxic. Other than wearing safety glasses and a lab coat, state one safety precaution that must be taken when carrying out this experiment. ..................................................................................................................................... [1] (c) Table 1.2 shows the student’s results. Table 1.2 temperature of water-bath / °C 100.0 laboratory atmospheric pressure / kPa 99.0 mass of conical flask, aluminium foil and rubber band 31.123 measured in step 2 / g mass of conical flask, aluminium foil, rubber band and 31.429 Y measured in step 7 / g mass of Y in flask in step 7 / g 0.306 volume of conical flask used / cm3 129.56 (i) Calculate the percentage error in the mass measured in step 2. Show your working. percentage error = .............................. [1] (ii) The student assumes that the vapour formed from liquid Y is an ideal gas. The ideal gas equation is shown. pV = nRT p = pressure of gas measured in Pa V = volume of gas in m3 n = number of moles of gas R = molar gas constant T = temperature in K Use the ideal gas equation and the results in Table 1.2 to calculate the amount, in mol, of Y in the conical flask in step 7. Assume that the temperature of the vapour formed from liquid Y is 100 °C. amount of Y = .............................. mol [2] (iii) Calculate the Mr of Y. Mr of Y = .............................. [1] (d) The actual temperature of the vapour formed from liquid Y is less than 100 °C. Describe and explain the effect this has on the calculated value of the Mr . effect on calculated Mr .............................................................................................................. explanation ............................................................................................................................... ................................................................................................................................................... [1] (e) The boiling point of methylbenzene is 111 °C. Suggest why the Mr of methylbenzene cannot be determined using this method. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 12]

Mark scheme: Question Answer Marks 1(a)(i) 0.9976 (g cm–3) 1 1(a)(ii) M1 mass of water = 161.175 – 31.022 g = 130.153 g 2 M2 volume = mass of water / density in (a)(i) = 130.153 / 0.9976 = 130.466 (cm3) 1(b)(i) to ensure the vapour (Y) (in the conical flask) has reached the temperature (of the boiling water / 100 °C) 1 1(b)(ii) mass of vapour (occupying flask)(at step 7) is used in calculation 1 1(b)(iii) (perform the experiment in a) fume hood 1 1(c)(i) ( 0.0005  2 ) 100 1 = 3.21  10–3 (%) 31.123 1(c)(ii) M1 correct value and units used for each quantity 2 • 99000 Pa • 1.2956  10–4 m3 (129.56  10–6) • 373 K (273 +100) M2 rearranges equation to give n=pV / RT AND correct evaluation 99000 129.56 10 −6 n = 8.31373 = 4.138055  10–3 (mol) 1(c)(iii) Mr = 0.306 / (c)(ii) = 73.94778 1 1(d) (calculated Mr would be) greater (than true value) 1 AND mass of vapour measured is greater (all other measurements are the same) 1(e) temperature (of water bath) not high enough (to boil methylbenzene) 1

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2 A student carries out an experiment to determine the charge of an aqueous ion, Mn+(aq), of metal M. The student prepares 100.0 cm3 of 1.00 mol dm–3 aqueous copper(II) nitrate, Cu(NO3)2(aq), to use in the experiment. (a) Calculate the mass of Cu(NO3)2•3H2O(s) required to prepare 100.0 cm3 of 1.00 mol dm–3 Cu(NO3)2(aq). mass = .............................. g [1] (b) Describe the steps the student should take to prepare 100.0 cm3 of 1.00 mol dm–3 Cu(NO3)2(aq) starting from the mass calculated in (a) supplied in a small beaker. Give the name and capacity of any apparatus used. Write your answer using a series of numbered steps. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (c) The student sets up the electrochemical cell shown in Fig. 2.1 to investigate the effect of changing the concentration of Mn+(aq) on the measured cell potential, Ecell. high-resistance voltmeter V A M(s) electrode Cu(s) electrode Mn+(aq) 1.00 mol dm–3 Cu(NO3)2(aq) Fig. 2.1 Suggest the function of the item labelled A in Fig. 2.1. ................................................................................................................................................... ............................................................................................................................................. [1] (d) The student uses the apparatus in Fig. 2.1 to measure the cell potentials, using six solutions each with a different concentration of Mn+(aq). Table 2.1 shows the results obtained by the student. Table 2.1 electrode concentration cell potential, potential of of Mn+(aq) log [Mn+] Ecell / V Mn+(aq) / M(s) / mol dm–3 / V 1.00 × 10–2 3.295 –2.00 5.00 × 10–3 3.304 –2.30 1.00 × 10–3 3.325 –3.00 5.00 × 10–4 3.343 –3.30 1.00 × 10–4 3.354 –4.00 1.00 × 10–5 3.384 –5.00 electrode potential of Mn+(aq) / M(s) = ECu – Ecell electrode potential of Cu2+(aq) / Cu(s), ECu = 0.337 V Complete Table 2.1. Record your values to three decimal places. [1] (e) Identify the independent variable in this experiment. ............................................................................................................................................. [1]

Mark scheme: 2(a) 24.15 (g) 1 2(b) M1 add a (small) volume of (distilled) water (to the small beaker) 3 AND dissolve the solid / Cu(NO3)2⦁3H2O(s) M2 transfer the solution to a 100 cm3 volumetric flask with washings M3 make up to the (calibration) mark / line (with distilled water) AND invert the (stoppered) flask (to mix) 2(c) to allow ion(s) to move (between half-cells / two beakers) 1 2(d) 1 log [Mn+] electrode potential of Mn+(aq) / M(s) / V –2.00 –2.958 –2.30 –2.967 –3.00 –2.988 –3.30 –3.006 –4.00 –3.017 –5.00 –3.047 2(e) concentration of Mn+ ((aq)) 1 2(f)(i) M1 points plotted correctly 2 M2 straight line of best fit drawn 2(f)(ii) point most anomalous to the plotted line of best fit circled 1 AND the actual concentration of the solution was lower than recorded value (5.00  10–4) 2(f)(iii) repeat the procedure for the solution giving an anomalous result 1 2(g)(i) M1 two acceptable coordinates from line of best fit expressed in the form (x,y) 2 M2 gradient correctly calculated from points listed for M1 to three significant figures 2(g)(ii) y = E 3 m = 2.303RT / nF c = E⦵ 2(g)(iii) M1 correct rearrangement AND values 2 2.303  8.31 298 n = gradient  9.65 10 4 M2 correctly calculated value rounded to whole number

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