It is possible to measure the rate at which potassium manganate(VII), KMnO₄(aq), Mᵣ = 158, diffuses through a permeable gel using the following method. [Figure 2.1] step 1 A petri dish is prepared with a permeable gel. step 2 A hole of diameter 0.5 cm is cut in the centre of the permeable gel. step 3 A sample of KMnO₄(aq) is placed into the hole and at the same time a stopwatch is started. step 4 After 3 minutes the diameter of the coloured spot is measured and recorded. step 5 The diameter is measured every 3 minutes until there are three successive equal measurements. A student obtained the results shown in Table 2.1. Table 2.1: time /minute | diameter of the coloured spot/cm | diameter increase of the coloured spot/cm 0 | 0.5 | 0.0 3 | 1.1 | 0.6 6 | 1.7 | 1.2 9 | 2.3 | 1.8 12 | 2.7 | 2.2 15 | 3.1 | 2.6 18 | 3.2 | 2.7 21 | 3.7 | 3.2 24 | 3.9 | 3.4 27 | 4.0 | 3.5 30 | 4.1 | 3.6 33 | 4.1 | 3.6 36 | 4.1 | 3.6

1. (a)Plot a graph on the grid to show the relationship between diameter increase of the coloured spot and time. Use a cross (x) to plot each data point. Draw a line of best fit.[2]
2. (b)(i)On the graph, circle the point which you believe to be the most anomalous.[1]
3. (b)(ii)Suggest a possible explanation for this anomaly.[1]
4. (c)Draw a suitable tangent to the line at time = 15 minutes. Calculate the gradient of your tangent. State both sets of coordinates used in your calculation. The stated coordinates must be from your tangent. Give the gradient to three significant figures. coordinates 1 ...................................... coordinates 2 ...................................... gradient = ...................................... cm minute⁻¹[3]
5. (d)Select appropriate data from Table 2.1 and calculate the average rate of diffusion of KMnO₄(aq) in cm minute⁻¹. average rate of diffusion of KMnO₄ = ...................................... cm minute⁻¹[1]
6. (e)Identify the independent variable in this experiment.[1]
7. (f)Suggest how the experiment could be made to be more reliable.[1]
8. (g)(i)Another compound of potassium which is coloured is potassium dichromate(VI), K₂Cr₂O₇, Mᵣ = 294. This compound is corrosive when aqueous. It is possible to use the method described earlier to determine the rate of diffusion of K₂Cr₂O₇(aq). Predict how the graph obtained for K₂Cr₂O₇(aq) would differ from that obtained for KMnO₄(aq). Explain your answer.[2]
9. (g)(ii)Apart from temperature, state one variable which must be controlled when comparing the rate of diffusion of K₂Cr₂O₇(aq) and KMnO₄(aq).[1]
10. (h)(i)Other than wearing eye protection, state one safety precaution the student should take if they were to use potassium dichromate(VI).[1]
11. (h)(ii)Another student suggests that to compare the rates of diffusion between K₂Cr₂O₇ and KMnO₄ it would be easier to place solid crystals of each of these compounds into the holes in two petri dishes of permeable gel. Suggest two practical problems that this would cause.[2]

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