Question

A lithium flame has a characteristic red color due to emissions of wavelength 671 nm. What is the mass equivalence of 14.0 mol of photons of this wavelength (1 J = 1 kg·m2/s2)?

Answer 1

WL = 671 nm = 671*10^-9 m

mass equivalence of

n = 14 mol photon

apply

E = hc/WL

h = planck constant = 6.626*10^-34

c = speed of light = 3*10^8

E = energy per unit photon

then

E = (6.626*10^-34)(3*10^8)/ (671*10^-9 )

E = 2.96244*10^-19 J/photon

we have

14 mol photon = 14*6.022*10^23 photons = 8.4308*10^24 photons so

E total = E photon * n total = (8.4308*10^24 )( 2.96244*10^-19) = 2497573.9152J

Apply einsetin concept of energy and mas

E = m*c^2

since c = speed of light then

m = (E)/(c^2)

m = (2497573.9152)/((3*10^8)^2) = 2.7750*10^-11 kg