The energy of one mole of photons of radiation of wavelength 300nm...

$\lambda = 300 \times {10}^{- 9}m$

Energy of single photon $= \dfrac{hc}{\lambda}$

Energy of ${N}_{A}$ photons $= \dfrac{6 .02 \times {10}^{23} \times 6 .63 \times {10}^{- 34} \times 3 \times {10}^{8}}{300 \times {10}^{- 9}}$

$= 3 . 99 \times {10}^{5}J / mole = 399KJ / mole$