Ok, here goes:
If it takes 1 mole of F2 158kJ to break up, then one molecule of F2 would require a photon with this much energy:
(158x10^3 J)/6.022 x10^23 mol^-1 = 2.62 x 10^-19 J
We calculate the frequency of the photon like this:
E = hv
so
2.62 x 10^-19 J = 6.626x10^-34 Js x v
v = 2.62 x 10^-19 J / 6.626x10^-34 Js = 3.960 x 10^14 s^-1
Now we convert this to a wavelength for that photon:
c = lv (sorry: l = lambda)
l = c/v = 3x10^8 ms^-1 / 3.960 x 10^14 s^-1 = 7.575x10^-7 m = 757.5 nm
... which is the wavelength of a photon of red light.
RE: #chemistrychallenge 3