Cambridge A Level Chemistry 9701 — 2020 May/June Paper 5 · Variant 2

1 A student plans to carry out an experiment to find the relative molecular mass, Mr, of a soluble acidic gas, X, by finding the mass of a measured volume of gas X. Gas X can be prepared by the reaction between concentrated sulfuric acid and copper. Two methods of gas collection are available to the student, as shown. concentrated concentrated gas sulfuric acid sulfuric acid syringe measuring cylinder copper copper gas Xturnings turnings clamp water heat heat beehive water shelf trough method A method B (a) (i) Explain why the first sample of gas collected from either apparatus should not be used for the Mr determination of gas X. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Give two reasons, other than your answer to (a)(i), why, for this experiment, the apparatus in method A is less suitable than the apparatus in method B for collecting a sample of gas X. reason 1 .............................................................................................................................. ............................................................................................................................................. reason 2 .............................................................................................................................. ............................................................................................................................................. [2] (b) The student is told to use the U-tube shown to find the mass of a sample of gas X. gas X U-tube gas syringe soda lime A 100.0 cm3 sample of pure gas X is placed in a gas syringe. The gas syringe is attached to a U-tube containing small lumps of solid soda lime, a mixture of sodium hydroxide and calcium hydroxide. All of gas X is slowly passed into the U-tube and the mass of gas X absorbed determined. The temperature and the pressure of the room are recorded. (i) State the measurements that are needed to determine the mass of gas X absorbed. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Suggest why soda lime is used to absorb gas X. ............................................................................................................................................. ....................................................................................................................................... [1] (c) Gas X can cause respiratory distress. State an appropriate precaution that should be taken when doing this experiment. .................................................................................................................................................... .............................................................................................................................................. [1] (d) Experiment 1 is carried out at a temperature of 21 °C, a pressure of 9.8 × 104 Pa, and uses 100 cm3 of pure gas X. (i) Calculate the number of moles of gas X present in Experiment 1. You should assume that gas X behaves like an ideal gas and so use PV = nRT. R = 8.31 J K–1 mol–1 moles of gas X = .............................. mol [2] (ii) The sample of gas X is found to have a mass of 0.251 g. Explain how the student should use this information and their results to determine the Mr of X. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Not all of gas X is absorbed by the soda lime. State what effect, if any, this has on the student’s calculated value of the Mr of gas X. ............................................................................................................................................. ....................................................................................................................................... [1] (iv) In Experiment 2, the same mass of gas X is used, but the student did not record the temperature and pressure. The calculated Mr of X for Experiment 2 is higher than the value calculated by the student for Experiment 1. P State and explain how the value of changes from Experiment 1 to Experiment 2. T ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (e) State how the reliability of the results in Experiment 1 could be improved. .................................................................................................................................................... .............................................................................................................................................. [1] (f) A different gas, methylamine, is alkaline. State a change that would have to be made to the apparatus so that the Mr of methylamine could be determined. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 13]

Mark scheme: 1(a)(i) The sample will contain air from the apparatus OR gas mixture not pure gas 1 1(a)(ii) M1 Gas is soluble so should not be collected over water 1 M2 There is a danger of suck-back 1 1(b)(i) Mass of U-tube + soda lime before the experiment AND Mass of U-tube + soda lime after the experiment 1 1(b)(ii) Soda lime is alkaline / a neutralisation reaction will occur. 1 1(c) Carry out the experiment in a fume cupboard 1 1(d)(i) M1 use of correct T (294 K) and V (100 × 10–6 m3) 1 M2 Correct use of pV = nRT (9.8 × 104 × 100 × 10–6 / 8.31 × 294) = 4.01 × 10–3 1 1(d)(ii) divide mass of X by the number of moles calculated in (d)(i) OR 0.251/ 1(d)(i) = 62.6 1 1(d)(iii) (Smaller mass of gas X absorbed, so) lower Mr value 1 1(d)(iv) P/T is less AND Because if Mr is higher, n is less and therefore P/T is less 1 1(e) Repeat the experiment (×3) and take an average 1 1(f) Replace the soda lime with an acid (to absorb the alkaline gas) 1

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2 Each student in a class of nine students performs an experiment to find the formula of a chloride of iron. Each student prepares the chloride of iron by passing a stream of chlorine gas over a sample of iron wire as shown in the diagram. excess chlorine gas iron wire dry chlorine gas chloride heat of iron (a) Each student carries out the following steps in the order shown in the list. Some of the measurement steps are missing from the list. ●● Weigh the reaction tube containing a quantity of iron wire. ●● Set up the apparatus as shown in the diagram. ●● Start the flow of dry chlorine gas. ●● Heat the iron wire until it has completely reacted. ●● Allow the apparatus to cool, with chlorine gas still flowing. ●● Weigh the side-arm conical flask containing the chloride of iron. (i) State the two additional measurement steps that each student must perform in order to find the formula of the chloride of iron. 1 .......................................................................................................................................... 2 .......................................................................................................................................... [1] (ii) The flow of dry chlorine gas must start before the iron wire is heated. Explain why. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) State an assumption that has to be made for the measurements made in this experiment to be valid. ............................................................................................................................................. ....................................................................................................................................... [1] (b) (i) The class results are shown in the table. mass of mass of mass of amount amount of student iron wire iron chloride chlorine of iron chlorine atoms / g / g / g / mol / mol 1 0.57 1.64 2 1.10 3.16 3 1.40 4.03 4 1.95 5.61 5 2.18 5.89 6 2.75 7.90 7 3.05 8.77 8 3.45 9.80 9 3.90 11.18 Calculate the mass of chlorine that reacts with the iron wire in each experiment. Record each mass to two decimal places. Calculate the amount of iron, in mol, and amount of chlorine atoms, in mol, that reacts in each experiment. Record the number of moles to three significant figures. [Ar: Fe, 55.8; Cl, 35.5] [3] (ii) Plot a graph on the grid of the amount of chlorine atoms against the amount of iron. Use a cross (×) to plot each data point. Draw a line of best fit through the plotted points. You should consider whether (0,0) should be on the line of best fit. [2] 3 am

Mark scheme: 2(a)(i) The mass of the empty reaction tube AND The mass of the empty side-arm conical flask 1 2(a)(ii) The iron could react with oxygen or air (instead of the chlorine) 1 2(a)(iii) any one from: • The mass of air in the flask is equal to mass of chlorine in the flask • All the iron (wool) has reacted • Any residue in the reaction tube is iron 1 2(b)(i) Student Mass of iron / g Mass of iron chloride / g Mass of chlorine / g Moles of iron Moles of chlorine 1 0.57 1.64 1.07 0.0102 0.0301 2 1.10 3.16 2.06 0.0197 0.0580 3 1.40 4.03 2.63 0.0251 0.0741 4 1.95 5.61 3.66 0.0349 0.103 5 2.18 5.89 3.71 0.0391 0.105 6 2.75 7.90 5.15 0.0493 0.145 7 3.05 8.77 5.72 0.0547 0.161 8 3.45 9.80 6.35 0.0618 0.179 9 3.90 11.18 7.28 0.0699 0.205 M1 Mol of iron (column 5) M2 Mol of chlorine (column 6) [ M3 all entries to 3 SF 3 Question Answer Marks 2(b)(ii) M1 5 points plotted correctly M2 Line of best fit drawn 2 Question Answer Marks 2(b)(iii) M1 Circle around the point at 0.0391,0.105 1 M2 Viable reason why the number of moles of chlorine is too small. e.g. not all the mass of iron chloride formed was collected in the conical flask OR Iron chloride dust particles escaped with excess chlorine gas 1 2(c) M1 Coordinates read and recorded correctly 1 M2 Gradient calculated 1 2(d) Calculate the moles of each element and find the mole ratio 1 2(e)(i) 2 x 0.005/4.03 x 100 = 0.248% 1 2(e)(ii) Lower percentage error AND Larger mass used 1 2(f)(i) (55.8/126.8) x 100 = 44.0 OR empirical formula calculation 44/55.8 : 56/35.5  0.7885 : 1.5775  1 : 2  FeCl2 1 2(f)(ii) In method (f) conversion of Fe to Fe2+ takes place as Eϴcell is + 0.44V AND Conversion of to Fe2+ to Fe3+ is not feasible as Eϴcell is – 0.77V 1

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