Crystal violet, C25H30N3Cl(s), is a purple dye. Some light is absorbed when it passes through C25H30N3Cl(aq). Absorbance is the proportion of light absorbed at a particular wavelength. This is measured using a colorimeter. A graph of absorbance against wavelength for C25H30N3Cl(aq) is shown in [Figure 2.1]. A student investigates how to determine the concentration of aqueous crystal violet, C25H30N3Cl(aq), using colorimetry.

1. (a)Suggest the best wavelength of light to use in the colorimeter when measuring the concentration of C25H30N3Cl(aq). wavelength = .................................... nm[1]
2. (b)(i)Calculate the mass of C25H30N3Cl(s) needed to prepare solution D. Give your answer to three significant figures. [Mr: C25H30N3Cl(s), 407.5] mass of C25H30N3Cl(s) = .................................... g[1]
3. (b)(ii)The student is given a small beaker containing the mass of C25H30N3Cl(s) calculated in (i). Describe how the student should prepare 500.0 cm³ of solution D. Include the name and capacity of the key apparatus which should be used and describe how the student should ensure the volume is exactly 500.0 cm³.[3]
4. (c)(i)Complete Table 2.1 to show the volumes of solution E and distilled water needed to prepare each of the solutions from 2 to 6. Give all volumes to two decimal places. Table 2.1 solution | volume of 2.50 x 10-4 mol dm-3 C25H30N3Cl(aq) (solution E) /cm³ | volume of distilled water /cm³ | [C25H30N3Cl(aq)] /moldm-3 | absorbance 1 | 0.00 | 20.00 | 0.00 | 0.000 2 | | | 0.50 x 10-4 | 0.191 3 | | | 1.00 x 10-4 | 0.270 4 | | | 1.50 x 10-4 | 0.545 5 | | | 2.00 x 10-4 | 0.711 6 | | | 2.50 x 10-4 | 0.860[1]
5. (c)(ii)Identify the dependent variable.[1]
6. (d)(i)Plot a graph of absorbance against [C25H30N3Cl(aq)] on the grid in [Figure 2.2]. Use a cross (x) to plot each data point. Draw a straight line of best fit.[2]
7. (d)(ii)Circle the point on the graph you consider to be most anomalous. Suggest one reason why this anomaly may have occurred during this experimental procedure. Assume no error was made in the measurement of absorbance.[2]
8. (d)(iii)State the relationship between [C25H30N3Cl(aq)] and absorbance.[1]
9. (d)(iv)Suggest how the student could improve the reliability of the data obtained in the experiment in (c).[1]
10. (e)(i)Suggest why it is not possible for the student to measure the absorbance of the mixture at t = 0 s.[1]
11. (e)(ii)Use the graph in [Figure 2.3] to find the half-life, t½, starting at 100s. State the coordinates of both points on the line of best fit used in your calculation. coordinates 1 .................................... coordinates 2 .................................... half-life .................................... s[2]
12. (e)(iii)Another student repeats the experiment at a different temperature and measures two half-life values. The values obtained are 420s and 425s. Use these values to deduce the order of the reaction with respect to C25H30N3Cl(aq). Explain your answer. order = .................................... explanation ...................................................................................................................[1]

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