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A-LevelChemistryStates of matter9701/51 · Oct/Nov 2020 · Q217 Marks

A student uses the apparatus shown to calculate the relative molecular mass, Mr, of a gaseous alkane. The experiment took place at 298K and 101kPa. [Apparatus diagram] The alkane is flammable. The student opens the tap and allows a small amount of the alkane gas into the measuring cylinder, displacing water. The gas in the measuring cylinder is allowed to reach room temperature and the volume recorded. This process is repeated and the measurements of mass of gas cylinder and total volume of gas collected are recorded in the table.

  1. (a)Complete column C in the table. The first reading has been done for you. Give all values to two decimal places.[1]
  2. (b)(i)Plot a graph on the grid of total mass of gas lost against the total volume of gas collected. Use a cross (x) to plot each data point. Draw a line of best fit.[2]
  3. (b)(ii)Circle the point which you think is most anomalous on your graph.[1]
  4. (b)(iii)Suggest one reason that explains the anomalous result you have circled.[1]
  5. (c)(i)Use the 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. Give the gradient to three significant figures.[2]
  6. (c)(ii)The ideal gas equation states that pV = nRT, where R = 8.31 Jmol⁻¹K⁻¹. Use the ideal gas equation and your answer to (c)(i) to calculate an experimental value for the Mr of the alkane gas. Give your answer to one decimal place. Show your working.[2]
  7. (c)(iii)Suggest the identity of the alkane gas. Explain your suggestion.[1]
  8. (d)(i)Suggest two ways in which the accuracy of the measurements may be improved.[2]
  9. (d)(ii)Apart from wearing eye protection or a lab coat, state one safety precaution which the student must take when carrying out this experiment.[1]
  10. (d)(iii)The student is told that samples of the alkane gas are sometimes contaminated with a more volatile alkane. State and explain how this would affect the gradient of the graph.[2]
  11. (d)(iv)In each experiment the gas was allowed to reach the temperature of the room (298K) before the volume was measured. Explain why this was necessary for the calculation of the Mr of the alkane.[1]
  12. (d)(v)Suggest how the reliability of the experiment could be improved.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

  1. [1] 2(a)(i) 0.25, 0.37, 0.54, 0.76, 1.00, 1.30, 1.64, 1.91, 2.19
  2. [2] 2(b)(i) M1: all ten points plotted correctly
  3. M2: straight line passing through 0,0
  4. [1] 2(b)(ii) 8th point circled 720,1.64
  5. [1] 2(b)(iii) (volume of gas too high and) could be caused by volume being measured at a temperature higher than 298 K OR (total mass of gas lost too low and) could be caused by mass reading taken before the tap closed
  6. [2] 2(c)(i) M1: coordinates must be in the format x,y and lie on the line of best fit
  7. M2: gradient calculated correctly with positive value
  8. [2] 2(c)(ii) M1: Manipulation of pV = nRT to include Mr in any expression, e.g. Mr = m/V × RT/p or Mr = gradient × RT/p OR partial / full substitution of data for symbols
  9. M2: Correct use of units and pV = nRT to a numerical answer to 1 dp
  10. [1] 2(c)(iii) Butane / methylpropane AND because C4H10 gives Mr close to calculated value in (c)(ii)
  11. [2] 2(d)(i) M1: Use a balance which records to more than 2 d.p.
  12. M2: Use a measuring cylinder (or gas syringe) AND with smaller divisions / greater resolution
  13. [1] 2(d)(ii) keep the collected gas / pressurised cylinder away from sources of ignition
  14. [2] 2(d)(iii) M1: the gradient would be smaller/less/less positive
  15. M2: (As 'Mr' is lower), for a particular mass, volume would be greater (so the gradient / density would be lower) OR (As 'Mr' is lower), for a particular volume, mass would be lower (so the gradient / density would be lower)
  16. [1] 2(d)(iv) Temperature change will alter gas volume OR Temperature must be constant to allow comparison between results
  17. [1] 2(e)(v) the experiment could be repeated and a mean value obtained

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About this question

Topic
States of matter, A-Level Chemistry (syllabus 9701). Structured question, 17 marks.
Source
A-Level Chemistry, Oct/Nov 2020 Paper 5 Variant 1, question 2.

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