What is the energy of 1 mole of light that has wavelength 400nm? ... ?

1 mole of light contains 6.02 x 10²³ photons

Energy of a given wavelength of light is given by:

E = nhc/λ; where n is the number of photons, h is the Planck's constant, c is the speed of light and λ is the wavelength.

E = (6.02 x 10²³ x 6.63 x 10⁻³⁴ x 3 x 10⁸) / (400 x 10⁻⁹)

E = 299.3 kJ