Find the energy of a photon whose frequency is 5 x 10^12 Hz

E = hv where h= Planck's constant = 6.626 x 10-²7 erg sec, and v is the frequency.

So the energy is (6.626 * 10^-27 erg sec)(5 * 10^12 sec^-1) = 3.313*10^-14 erg.