What are the units of molar absorptivity, ε?

• A. L·mol−1·cm−1
• B. S·mol−1·cm−1
• C. Cm^2·mol−1
• D. L·cm−1

Answer: (A)

The units of molar absorptivity come from the Beer–Lambert law, A = ε l c. Absorbance A is dimensionless, l is the path length in centimeters, and c is the concentration in moles per liter. To keep A dimensionless, ε must have units that cancel both the concentration and the length, giving ε units of L·mol−1·cm−1. Using M−1·cm−1 is equivalent because M ≡ mol/L, but the standard expression is L·mol−1·cm−1. The other suggested units don’t cancel both the concentration and the length, so they wouldn’t yield a dimensionless absorbance.