Effusion is the process in which a gas escapes through a small hole. A student investigates the relationship between rate of effusion and relative molar mass of a gas using the apparatus shown in Fig. 2.1. [Figure 2.1] The following method is used: step 1 Turn the tap and remove any gas from the syringe through the side-arm tube, by pushing in the plunger. step 2 Add 50 cm³ of the gas being tested to the syringe through the side-arm tube. step 3 Remove the gas from the syringe, through the side-arm tube, by pushing in the plunger. step 4 Add 70 cm³ of the gas being tested to the syringe through the side-arm tube. step 5 Turn the tap to connect the syringe to the tube with the aluminium foil and small hole. step 6 Allow the syringe plunger to fall and start a timer when the volume of gas in the syringe reaches 60 cm³. step 7 Stop the timer when the volume of gas in the syringe reaches 10cm³. Record the time taken. step 8 Repeat steps 1 to 7 with different gases.

1. (a)Suggest why the student adds 50 cm³ of the gas being tested to the syringe in step 2 and then removes this gas in step 3.[1]
2. (b)The student's results are shown in Table 2.1. Table 2.1 | gas | hydrogen, H₂ | helium, He | neon, Ne | argon, Ar | krypton, Kr | | :------------------- | :---------- | :---------- | :-------- | :--------- | :----------- | | relative molar mass, M | 2.0 | 4.0 | 20.2 | 39.9 | 83.8 | | 1/√M | | | | | | | time taken /s | 10.8 | 15.3 | 34.5 | 39.7 | 70.4 | | rate of effusion /cm³s⁻¹ | | | | | | rate of effusion = volume of gas / time taken
3. (b)(i)Complete Table 2.1. Give the values for 1/√M to three significant figures. Give the values for rate of effusion to two decimal places.[2]
4. (b)(ii)Identify the dependent variable in this experiment.[1]
5. (b)(iii)Identify a variable, other than temperature, that is controlled when carrying out this experiment.[1]
6. (c)Plot a graph on the grid in Fig. 2.2 to show the relationship between rate of effusion and 1/√M. Use a cross (x) to plot each data point. Draw a suitable line of best fit. [Figure 2.2][2]
7. (d)Circle one point on the graph in Fig. 2.2 which you consider to be most anomalous. Suggest one reason for this anomaly. Assume there is no error in 1/√M.[1]
8. (e)Graham's law of effusion can be expressed as: the rate of effusion of a gas is proportional to 1/√M. State whether or not the student's results support Graham's law of effusion. Explain your answer, using the graph in Fig. 2.2.[1]
9. (f)Suggest how the position of the plotted points relative to the line of best fit in Fig. 2.2 is related to the reliability of the results.[1]
10. (g)The student then repeats this method to determine the value of M of a sample of natural gas. The time recorded in step 7 is 31.6 s.
11. (g)(i)Use the graph in Fig. 2.2 and the student's result to calculate the value of M for this sample.[2]
12. (g)(ii)Natural gas is a mixture of mainly methane, CH4, with small amounts of other gases. Suggest what your calculated value of the M of natural gas in (g)(i) tells you about the other gases in the mixture.[1]
13. (h)The experiment described in (g) is repeated at a higher temperature. Suggest how the rate of effusion for this sample of natural gas would change, if at all. Explain your answer. effect on the rate of effusion explanation[1]

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