Which equation expresses absorbance in terms of percent transmittance?

• A. A = abc
• B. A = 2-log%T
• C. A = log (100/%T)
• D. Abs of unknown / Abs of standard = Conc of unknown/ Conc of standard

Answer: (C)

Absorbance is defined as the logarithm of the reciprocal of the transmitted fraction. If %T is the percent transmittance, then the transmitted fraction is T = %T/100. So A = -log10(T) = -log10(%T/100) = log10(100/%T). This directly expresses absorbance in terms of percent transmittance. Since log10(100/%T) can also be written as 2 − log10(%T) (because log10(100) = 2), the same relationship can be shown in that form as well. The direct form is the standard way to relate absorbance to percent transmittance, which is why A = log(100/%T) is the correct expression.