In Beer's Law A = a b c, which statement is correct?

• A. a = molar absorptivity; b = light path / cuvet; c = concentration
• B. a = light path; b = molar absorptivity; c = concentration
• C. a = concentration; b = absorbance; c = path length
• D. a = absorbance; b = concentration; c = path length

Answer: (A)

Beer's Law shows how absorbance depends on how strongly the substance absorbs light, how far the light travels through the sample, and how much solute is present. In A = a b c, the symbol for molar absorptivity is a, the path length the light travels through the sample is b (the cuvet’s length), and the concentration of the solute is c. This pairing makes sense because increasing any of these factors increases absorbance: a substance with higher absorptivity interacts more with the light, a longer path length gives more interaction, and a higher concentration provides more absorbing molecules. The units line up so that epsilon (a) has units of L/(mol·cm), path length is in cm, and concentration is in mol/L, giving A as a dimensionless quantity.