The energy of one mole of photons of radiation of frequency 2 × 10...

For one photon, from Planck's equation, $E = hv$

For one mole photon,

$⇒ E = 6 \cdot 023 \times {10}^{23} \times 6 \cdot 626 \times {10}^{- 34} \times 2 \times {10}^{12}$

$= 798 \cdot 16J$

$\approx 798J$