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Question

In a measurement of quantum efficiency of photosynthesis in green plants, it was found that 10 quanta of red light of wavelength 6850 A were needed to release one molecule of O2. The average energy storage in this process is 112 kcal/mol of O2 evolved. The energy conversion efficiency in this experiment is:

A
23.75%
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B
26.90%
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C
66.23%
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D
73.19%
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Solution

The correct option is B 26.90%
According to Planck’s quantum theory E=hcλ
where E = energy of 1 quanta of the radiation
h = Planck's constant =6.626×1034 J s
c = speed of light =3×108 m/s
λ = wavelength of the radiation =6850 A=6850×1010 m
So, E=6.626×1034×3×1086850×1010=2.9×1019J

Energy of 10 quanta =10×2.9×1019=29×1019 J

Given: average energy stored =112 kcal/mol of O2
We know 1 calorie = 4.18 J
Therefore, energy of 1 molecule =112×4.18×1036×1023=7.80×1019 J

% efficiency =7.80×101929×1019×100=26.90%

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