When light passes through solutions of chemical compounds some of the light may be absorbed. The quantity of light absorbed is called the absorbance and it is measured by a spectrophotometer. A simplified diagram of a spectrophotometer is shown. A glass cuvette is a rectangular vessel. [Figure of spectrophotometer showing light source, sample in cuvette, and light detector]

1. (a)(i)A chemist placed distilled water in the glass cuvette. This was then put into the spectrophotometer and a reading taken. Explain why this reading was taken.[1]
2. (a)(ii)Light passes through opposite sides of the cuvette. These two sides must be wiped with a cloth to ensure they are clean and dry. Explain why this procedure makes the readings more accurate.[1]
3. (b)(i)1.0 dm³ of a standard solution of 0.0300 mol dm⁻³ MnO4⁻ was prepared by a chemist using solid potassium manganate(VII), KMnO4, measured using a two decimal place balance. Calculate the mass of KMnO4 required to prepare this standard solution. [A: K, 39.1; Mn, 54.9; O, 16.0] mass of KMnO4 =[1]
4. (b)(ii)Describe how the chemist should accurately prepare this standard solution using a sample of KMnO4 of mass calculated in (i). There is a 1.0 dm³ volumetric flask available.[2]
5. (b)(iii)The chemist diluted this standard solution to 3.0 × 10⁻⁴ mol dm⁻³ for use in the experiments. Explain why the chemist did not prepare a solution of this concentration directly, by dissolving the required mass of KMnO4 in 1.0 dm³ of water.[1]
6. (c)The chemist needed to determine which wavelength of light was most absorbed by a solution of MnO4⁻ (aq). The clean, dry cuvette was filled with 3.0 × 10⁻⁴ mol dm⁻³ MnO4⁻ and different wavelengths of light were passed through the solution. A graph of the results was plotted. [Graph showing absorbance vs wavelength/nm] Use the graph to estimate the wavelength of light that is most absorbed by the MnO4⁻ solution. wavelength of light most absorbed = nm[1]
7. (d)(i)The spectrophotometer was then set to the wavelength that is most absorbed by the MnO4⁻(aq) solution. The chemist measured the absorbance of solutions of known concentrations of MnO4⁻ (aq). The results are shown in the table. concentration of MnO4(aq)/mol dm⁻³ | absorbance ---|--- 3.00 × 10⁻⁴ | 0.748 2.70 × 10⁻⁴ | 0.680 2.40 × 10⁻⁴ | 0.610 2.10 × 10⁻⁴ | 0.530 1.80 × 10⁻⁴ | 0.440 1.50 × 10⁻⁴ | 0.378 1.20 × 10⁻⁴ | 0.315 0.90 × 10⁻⁴ | 0.230 0.60 × 10⁻⁴ | 0.150 Plot a graph on the grid on page 11 to show the relationship between the absorbance and the concentration of MnO4⁻(aq). Use a cross (x) to plot each data point. Draw a line of best fit.[2]
8. (d)(ii)State the relationship between absorbance and concentration of MnO4⁻(aq). Explain your answer with reference to particles.[2]
9. (d)(iii)Do you consider the results obtained to be reliable? Explain your answer.[1]
10. (e)(i)The chemist used the MnO4⁻(aq) solution of concentration 3.00 × 10⁻⁴ mol dm⁻³ to prepare the solutions in the table on page 10. Calculate the volume of 3.00 × 10⁻⁴ mol dm⁻³ MnO4⁻(aq) solution and the volume of distilled water required to prepare a 25.00 cm³ solution of 2.70 × 10⁻⁴ mol dm⁻³ MnO4⁻(aq). Give your answers to two decimal places. volume of 3.00 × 10⁻⁴ mol dm⁻³ MnO4⁻(aq) solution = volume of distilled water =[1]
11. (e)(ii)The volumes of the two solutions given in (e)(i) could be measured using the same type of apparatus. Name a suitable piece of apparatus which could be used to measure these volumes.[1]
12. (f)(i)The chemist dissolved a known mass of steel, containing manganese, in acid. The manganese was then oxidised to manganate(VII) ions, MnO4⁻, using a very strong oxidising agent. The resulting solution was made up to 100.0 cm³ in a volumetric flask. The absorbance of the MnO4⁻(aq) solution was 0.630. Use the graph you have drawn in (d)(i) to determine the concentration of MnO4⁻(aq) in this solution. Give your answer to three significant figures. concentration of MnO4⁻(aq) =[1]
13. (f)(ii)Calculate the mass of manganese present in the steel sample. Show your working. [Ar: Mn, 54.9] mass of manganese =[1]
14. (g)The steel sample that the chemist used had a mass of 1.209g. Use the mass of manganese you calculated in (f)(ii) to calculate the percentage of manganese by mass that was present in the steel sample. (If you were unable to calculate an answer to (f)(ii) you may use 0.00143g as the mass of manganese. This is not the correct answer.) percentage of manganese in the steel sample =[1]
15. (h)Another way of analysing the manganese content of the steel sample is by titration. The steel sample is prepared in the same way as previously. It is dissolved in acid and then oxidised using a very strong oxidising agent. The MnO4⁻(aq) ions produced are titrated with a solution of iron(II) ions. The equation for this reaction is shown. MnO4⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H2O(l) Explain why it is essential to remove the strong oxidising agent used to prepare the solution of steel sample before carrying out the titration.[1]

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