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How to Calculate Molar Absorptivity

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By Allan Robinson
eHow Contributing Writer
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Molar absorptivity is also known as the molar extinction coefficient and is a measure of well a chemical species (chemically identical molecules) absorb a given wavelength of light. It is commonly used in chemistry and should not be confused with the extinction coefficient, which is used more often in physics. The standard units for molar absorptivity are square meters per mole but it is usually expressed as square centimeters per mole.

Difficulty: Moderate
Instructions

Things You'll Need:

  • Calculator

    The Steps

  1. 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 absorbed by the solution. The concentration c of the absorbing species is the amount of absorbing species per unit volume. The path length l is the distance that the light travels through the solution. We will call the molar absorptivity e.

  2. Step 2

    Use the Beer-Lambert Law to calculate the molar absorptivity of a single absorbing species. This is given as A = ecl, so we can express the molar absorptivity as e = A/cl.

  3. Step 3

    Calculate the total absorbance of a solution that contains more tha one absorbing species. We can expand the Beer-Lamber law to A = (e1c1 + e2c2 + ...)l where ei is the molar absorptivity of species i and ci is the concentration of species i in the solution.

  4. Step 4

    Calculate the molar absorptivity from the absorption cross section and Avogadro's Number (approximately 6.022 x 10^23). We have d = (2.303/N) e where d is the absorption cross section and N is Avogadro's Number. Therefore, d = (2.303/(6.022 x 10^23)) e = 3.82 x 10^(-21) e, so e = (2.62*10^20)d.

  5. Step 5

    Predict the molar absorptivity of light at 280 nm by a protein. The molar absorptivity under these conditions depends almost entirely on the number of aromatic residues that the protein has, especially tryptophan.

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