How to Calculate Molar Absorptivity

Molar absorptivity is a measure of how well chemically identical molecules absorb a given wavelength of light.

Molar absorptivity, also known as the molar extinction coefficient, is a measure of how well a chemical species (chemically identical molecules) absorbs 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.

Things You'll Need

• Calculator

Instructions

1. Define the variables needed to calculate the molar absorptivity. The absorbance (A) is the amount of light within 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. The molar absorptivity is represented by "e."

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

3. Calculate the total absorbance of a solution that contains more than one absorbing species. 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. Calculate the molar absorptivity from the absorption cross-section and Avogadro's Number (approximately 6.022 x 10^23); 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. Predict the molar absorptivity of light at 280 nm by a protein. The molar absorptivity under these conditions depends on the number of aromatic residues the protein has, especially tryptophan.

References

• Beer's Law

Resources

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