Things You'll Need:
- Calculator
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Step 1
Define the variables needed to calculate the molar absorptivity. The absorbance A is the amount of light with a given wavelength that is blocked by the solution. The concentration c of the chemical species is the quantity of absorbing species per unit volume. The path length l is the distance that the light travels through the solution.
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Step 2
Use the Beer-Lambert Law to calculate the molar absorptivity e of a single absorbing species. This is given as A = ecl, therefore e = A/cl.
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Step 3
Determine the units of measure for the molar absorptivity. A is a unitless measure, c is measured in moles/volume and l is a length. Therefore, the units of measure for molar absorptivity are 1/(moles length)/volume or area/mole. We can therefore see that e will be in units of area per unit mole, typically square meters/mole.
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Step 4
Calculate the molar absorptivity from the absorption cross section and Avogadro's Number. We have d = (2.303/N) e where d is the absorption cross section and N is Avogadro's Number. Thus, e = Nd/2.303 and N is approximately 6.022 x 10^23 so e = d, so e = (6.022 x 10^23)d/2.303 = 2.62*10^20 d. Therefore, we have e = 2.62*10^20 d.
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Step 5
Calculate the molar absorptivity e from the mass attenuation coefficient u and the molar mass m of a chemical species. This relationship may be expressed as e = um.
