State the Beer-Lambert law and define A, ε, l, and c.

• A. A = εlc; A is absorbance, ε is molar absorptivity, l is path length (cm), c is concentration (mol/L)
• B. A = εlc; A is concentration, ε is molar absorptivity, l is path length (cm), c is absorbance
• C. A = εlc; A is absorbance, ε is molar absorptivity, l is path length (m), c is concentration (mol/L)
• D. A = εlc; A is absorbance, ε is energy, l is path length (cm), c is concentration (mol/L)

Answer: (A)

The Beer-Lambert law describes how the amount of light absorbed by a solution depends on how much solute is present and how far the light travels through the sample, scaled by how strongly the solute absorbs at that wavelength. The relation is A = ε l c. Absorbance, A, is defined as A = log10(I0/I) and is unitless. ε is the molar absorptivity (extinction coefficient), with units L mol^-1 cm^-1. l is the path length that light travels through the sample, measured in centimeters. c is the concentration, in moles per liter (mol/L). Using ε in those units with l in centimeters and c in mol/L makes the product εlc dimensionless, matching A. This is why the correct description uses A for absorbance, ε as molar absorptivity, l in cm, and c in mol/L. The other options mix up definitions or units (such as A representing concentration, or l in meters, or ε described as energy), so they don’t align with the standard form.