What is the energy of a mole of UV photons having a wavelength of 39 nm?

Question

A photon of ultraviolet (UV) light possesses enough energy to mutate a strand of human DNA. What is the energy of a single UV photon having a wavelength of 39 nm?
5.1e-18 J

that was the first question...but then it asks:
What is the energy of a mole of UV photons having a wavelength of 39 nm?
-the answer is in Joules


please explain it!

Answer 1

If you know the energy of one photon, multiplying by Avogadro's Number will give you the energy of a mole of photons of the same kind.

You may not be used to the idea of a mole of photons, but you can have a mole of anything. By the way, a mole of photons is called an Einstein.