Question

A 3.00 mL ampule of 0.120 M solution of naphthalene in hexane is excited with a flash of light. The naphthalene emits 14.2 J of energy at an average wavelength of 349 nm. What percentage of the naphthalene molecules emitted a photon?

Answer 1

E = hc/λ

E = 6.626*10^-34 x 2.998*10^8 / 349*10^-9

E = 5.692*10^-19J

number of the photons emitted
14.2 / 5.692*10^-19 = 2.49 x 10^19 Photons

moles of nepthalene
3 x 0.120 /1000 =3.6 x 10^-4 mol

number of its molecules
3.6 x 10^-4 x 6.022*10^23 =2.17 x 10^20 molecules

percentage of the naphthalene molecules = (2.49 x 10^19 / 2.17 x 10^20 ) x100
                                                                 =11.48 %