(solved)Question 2.8 of NCERT Class XI Chemistry Chapter 2

by Planck's formula for energy of n photons

$\displaystyle E = n \times h\nu = n \times \frac{hc}{\lambda} \text{ (J)}$

$\displaystyle \implies n = \frac{E \times \lambda}{hc}$

where h, c, $\displaystyle \lambda\text{, }\nu$ mean as usual

(1) Put $\displaystyle \lambda = 4000 \text{ pm} = 4000 \times 10^{-12} \text{m}$

(2) Put $\displaystyle c = 3 \times 10^8 \text{m/s}$

(3) Put $\displaystyle h = 6.6 \times 10^{-34} \text{ Js}$

(4) Put $\displaystyle E = 1\text{ J}$

$\displaystyle n = \frac{1 \times 4000 \times 10^{-12}}{6.6 \times 10^{-34} \times 3 \times 10^8}$

$\displaystyle = 2.02 \times 10^{16}\text{ photons }\underline{Ans}$