Question

In a measurement of quantum efficiency of photosynthesis in green plants, it was found that $10$ quanta of red light of wavelength $6850\mathring{A}$were needed to release one molecule of $O_{2}$. The average energy storage in this process for $1$ mole $O_{2}$ evolved is $112$ kcal.
What is the energy conversion efficiency in this experiment?
[Given: $1 cal = 4.18 J; N_{A} = 6 \times 10^{23}; h = 6.63 \times 10^{-34}$ Js]

Answer 1

$E =\frac{hc}{\lambda} = 2.9 \times10^{-19}J$
Total energy of $10$ quanta
$\Rightarrow 10 \times2.9 \times10^{-19}$
$\Rightarrow 29\times10^{-19}J$
Energy stored for process
$= \frac{112 \times4.18 \times10^{3}}{6 \times10^{23}} = 7.80 \times10^{-19} J$
$\%$ efficiency $= \frac{7.8\times10^{-19}}{29 \times10^{-19} }\times100$
$\Rightarrow 26.9\%$