Question

A 3.00 mL ampule of a 0.120 M solution of naphthalene in hexane is excited with a flash of light. The naphthalene emits 10.8 J of energy at an average wavelength of 349 nm.

What percentage of the naphthalene molecules emitted a photon?

Answer 1

From E= hC/wave length gives energy of single photon

h= plancls constant= 6.627*10^-34J.s. C= 3*10⁸ m/s Wave length= 349*10-9m

E= 6.627*10^-34*3*10⁸/ 349*10^-9=5.69*10^-19 Joules/photon

number of photons= 10.8/(5.69*10^-19)= 1.9*10¹⁹ pjotons

Moles of napthalene in 3ml of 0.120M= 0,120*3/1000=0.00036 moles

1 mole of any substance contains 6.023*10²3 molecules

0.00036 moles of substance contains 6.023*0.00036*10²³ molecules=2.17*10²⁰ molecules

Fraction of molecules emiting photon= 100*(1.9*10¹⁹/2.17*10²⁰ )= 8.76%

Energy emitted= 10.8 Joules