We find the work function per atom e = E/N = 419000/6.023E23 = hf = photon energy; solve for the frequency, using N = 6.023E23 atoms per mole and Planck's h = 6.63E-34 J.s:
f = E/Nh = 419000/(6.023E23*6.63E-34) = 1.04927E+15 = 1.05E15 cps IS THE ANSWER, HOPE IT HELPED :)
