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 649 nm emission.

E = 1.8 x 10⁵ J/mol

Explanation:

We are being asked the enery per mol for an emission line corresponding to 649 nm.

The energy of a photon is given by the porduct of Planck's contant times the frequency of the radiation,

                       E = hν

We also know that  the frequency is given by

ν = c/λ

where c is the speed of light (3 x 10 ^8 m/s) and λ is 649 nm given in the problem. Therefore the energy per photon will be given by

E= hc/λ = 6.626 x 10⁻³⁴Js x 3 x 10 ^8 m/s/ 649 x 10 ⁻⁹ m

E = 3.1  x 10 ⁻¹⁹ J/ photon

(Note the wavelength has to be in nanometers  (1nm= 10⁻⁹ m) and that the energy we get is the energy per a single photon. Thus we will need to multiply this result by Avogadro's number to answer this question.

E = 3.1  x 10 ⁻¹⁹ J/ photon x 6.022 x 10 ²³photon/mol

E = 1.8 x 10⁵ J/mol

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