First use the formula, E = hc/λ
(6.626 x 10-34 Js x 3 x 108
ms-1)/ (349 x 10-9 m) = 5.695 x
10-19J
Number of the photons emitted =
(12.5 J) / (5.695 x 10-19J) photons
                                                 Â
= 2.195 x 10-19 photons
Moles of naphthalene = volume in (L) x molarity
                            Â
     = (0.001 x 0.150) mole
                            Â
     = (1.50 x 10-4) mole
Number of its molecules = Moles / Avogrado number
                                    Â
= (1.50 x 10-4) x (6.023 x 1023)
                                    Â
= 9.035 x 10-19 molecules
Percentage of the naphthalene molecules emitted a photon =
(2.195 x 10-19) / (9.035 x 10-19) x 100
                                                                                              Â
= 24.3%
