Example:
What is the wavelength
g ((in nanometers)) of uv light
g
with  = 5.5 x 1015 s-1?
ν = c/ λ , rearrange to give λ = c/ν

3.00 x108 m / s
5.5 x1015 / s
λ = 5.4 x 10-8 m = 54 x 10-9 m = 54 nm
Units OK for wavelength (distance),
is frequency correct for uv light?
Chem 101
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1
Example:
MRI body scanners operate with 400 MHz radio
frequency energy. How much energy does this
correspond to in kilojoules/mol?
Energy = h x (frequency) or E = h ν
hertz (Hz) is sec-1 and
M means mega (106)
Note: the equation gives the energy of one photon, problem
asks for energy of 1 mol of photons.
Will need to use Avogadro’s number to calculate energy
f one mol.
for
l
E = (6.63 x 10-34 J s) (400 x 106 s-1) = 265 x 10-27 J
= (265 x 10-27 J )(10-3kJ/J) = 265 x 10-30 kJ = 2.65 x 10-28 kJ per photon
Energy per mol = (E/photon) (photons/mol )
= (2.65 x 10-28kJ/photon)(6.023 x 1023photons/mol) = 1.60 x 10-4 kJ/mol
Units are correct, answer seems reasonable
Chem 101
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2