posted by marcus .
when Na compounds are heated, thy give off a light of 5890 A. If 1 x 10-4 mole of Na atoms each give off a photon of this wavelength, how many kilojoules of energy are given off?
Actually the lines given off by Na are a doublet; i.e., one at 5890 A and the other at 5896 A. But we won't quibble.
Convert 5890A to meters and substitute in
E = hc/wavelength.
Solve for E which will be joules per photon. That times 6.02E23 will give joules/mol and that x 1E-4 will give joules in 1E-4 mols. Convert that to kJ.