11. (a) A solution of K, [Fe(C,0)l of concentration 0.0060 M is contained in a c
ID: 554584 • Letter: 1
Question
11. (a) A solution of K, [Fe(C,0)l of concentration 0.0060 M is contained in a cell of thickness 20 mm and is irradiated with light of wavelength 400 nm. The molar absorption coefficient at this wavelength is 516 Mlcm1. What fraction of the incident light intensity is absorbed? 11. (b) The Fe(III) is reduced to Fe(I) on irradiation and the quantum yield for Fe (II) formation at wavelength of 400 nm is 0.87. If the solution is irradiated for 20 minutes with light of 400 nm, it is found that 1.36 x104 moles of Fe (II) are formed. What is the intensity of the incident light on the cell (measured in quanta s? Explain how this problern is connected to the idea of a chemical actinometer.Explanation / Answer
11 (a)
Given;
Molar absorption coefficient ( ) = 516 M-1cm-1
Path length (x) = 20 mm = 2 cm
Concentration (c) = 0.0060 M
Absorbance (A) = ?
By the Beer-Lambert equation;
A = cx
Time = 20 min, wavelength = 400 nm
Quantum yield () = 0.87
Moles of Fe(II) produced (x1) = 1.36 x 10-4
Quanta of light used (x2) = ?
By the formula;
= x1/x2
intensity in terms of quanta per sec = quanta irradiated/ time in second =
(1.5632 x 10-4)/ (20 x 60) = 0.130 x 10-6 quanta/s (answer)
The moles of Fe(II) generated can be quantified to know the intensity of light incident on the cell. infact, a chemical actinometer is a device used to determine the intensity of irradiation based on chemical reaction which is exactly what is happening herein.
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