CIE Jun 2017 v2 Paper 5 Q2

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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. PMT sample of solution light detector glass cuvette light of one wavelength (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] (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]UCLES 2017 9701/52/M/J/17<br /> Manganese is added to steel to increase its strength. A spectrophotometer can be used to analyse the manganese content of steel. This is done by comparing the absorbance of a solution of MnO4 (aq) prepared from a sample of steel, with the absorbance of solutions of known concentrations of MnO4 (aq). PMT (b) 1.0 dm3 of a standard solution of 0.0300 mol dm3 MnO4was prepared by a chemist using solid potassium manganate(VII), KMnO4, measured using a two decimal place balance. (i) Calculate the mass of KMnO4 required to prepare this standard solution. [Ar: K, 39.1; Mn, 54.9; O, 16.0] mass of KMnO4 =g [1] (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] (iii) The chemist diluted this standard solution to 3.0104 mol dm3 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]UCLES 2017 9701/52/M/J/17 [Turn over<br /> PMT e c n a b r o s b a (c) The chemist needed to determine which wavelength of light was most absorbed by a solutionand different of MnO4 wavelengths of light were passed through the solution. A graph of the results was plotted. (aq). The clean, dry cuvette was filled with 3.0104 mol dm3 MnO4 0.760 0.750 0.740 0.730 0.720 0.710 0.700 0.690 0.680 0.670 0.660 0.650 0.640 0.630 0.620 0.610 0.600 510 515 520 525 530 535 540 545 550 wavelength / nm Use the graph to estimate the wavelength of light that is most absorbed by the MnO4solution. wavelength of light most absorbed =nm [1]UCLES 2017 9701/52/M/J/17<br /> PMT Question 2 continues on the next page.UCLES 2017 9701/52/M/J/17 [Turn over<br /> 10 PMT (d) 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 The results are shown in the table. (aq). concentration of MnO4 (aq) / mol dm3 3.00104 2.70104 2.40104 2.10104 1.80104 1.50104 1.20104 0.90104 0.60104 absorbance 0.748 0.680 0.610 0.530 0.440 0.378 0.315 0.230 0.150 (i) Plot a graph on the grid on page 11 to show the relationship between the absorbance and the concentration of MnO4 Use a cross () to plot each data point. Draw a line of best fit. (aq). [2] (ii) State the relationship between absorbance and concentration of MnO4 answer with reference to particles. (aq). Explain your[2] (iii) Do you consider the results obtained to be reliable? Explain your answer.[1]UCLES 2017 9701/52/M/J/17<br /> 11 PMT 0.800 0.700 0.600 0.500 absorbance 0.400 0.300 0.200 0.100 0.000 0.00 0.50104 1.00104 1.50104 2.00104 2.50104 3.00104 3.50104 concentration of MnO4 (aq) / mol dm3UCLES 2017 9701/52/M/J/17 [Turn over<br /> (e) (i) The chemist used the MnO4 the solutions in the table on page 10. (aq) solution of concentration 3.00104 mol dm3 to prepare 12 PMT Calculate the volume of 3.00104 mol dm3 MnO4 water required to prepare a 25.00 cm3 solution of 2.70104 mol dm3 MnO4 Give your answers to two decimal places. (aq) solution and the volume of distilled (aq). volume of 3.00104 mol dm3 MnO4 (aq) solution =cm3 volume of distilled water =cm3 [1] (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] 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. (f) A small sample of the solution of MnO4 clean, dry cuvette and its absorbance measured using the spectrophotometer. (aq) prepared from the steel sample was placed into a (i) 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 solution. Give your answer to three significant figures. (aq) in this concentration of MnO4 (aq) =mol dm3 [1] (ii) Calculate the mass of manganese present in the steel sample. Show your working. [Ar: Mn, 54.9] mass of manganese =g [1]UCLES 2017 9701/52/M/J/17<br /> (g) The steel sample that the chemist used had a mass of 1.209 g. 13 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 0.00143 g as the mass of manganese. This is not the correct answer.) PMT percentage of manganese in the steel sample =% [1] (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] [Total: 18]UCLES 2017 9701/52/M/J/17 [Turn over<br />

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