A YAG laser is an extremely powerful laser. Each pulse of light contains 5.50 Joules of energy. If the wavelength is 1.064 um (micrometers), how many moles of photons are contained within each pulse?
According to the difination, the mole is defined as the amount of substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 0.012 kg of the isotope carbon-12. In the difination can we include the photons? What is the molecular weight of the photon?
From what Mr F said $\displaystyle E_p = \frac{hc}{\lambda}$ and hence there are $\displaystyle n = \frac{E_t}{E_p \cdot N_A}$ where $\displaystyle E_t$ is total energy (5.5J), $\displaystyle E_p$ is the energy of a proton and $\displaystyle N_A = 6.02 \cdot 10^{23}$ is the Avogadro constant.
We can say that a mole of photons has the Avogadro number of photons in. As for mass it becomes more complicated since photons have no rest mass but do tend to have relativistic mass as evidenced by pair production.