calculate the energy?

Question

The bright-red color of highway safety flares comes from strontium ions in salts such as Sr(NO3)2 and SrCO3. Burning a flare produces strontium ions in excited states, which then emit red photons at 606 nm and several wavelengths between 636 and 688 nm. Calculate the energy (in Joules per mole) of the 684-nm emission.

Answer 1

It seems to me that you have trouble in electro-photon reaction. Please memorize the formula between the energy and frequency or wavelength.
To convert energy to frequency or wavelength, we have to use the following formula:
E = h•ν = h•c/λ
where h the Plank constant 6.626x10^-34 J•s, ν the frequency in 1/s, c the speed of light 2.998x10^8m/s, and λ the wavelength in m (1nm = 10^-9m). Now:
E = (6.626x10^-34 J•s)*(2.998x10^8m/s)/(684x10^-9m)
= 2.904x10^-19 J
Now this energy is for one photon from one atom. For one mole:
6.022x10^23 * 2.904x10^-19 J = 175 kJ.