Multiple Choice
When analyzing an absorbance spectrum, which approach is most appropriate for identifying compounds present in a mixture?
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17. Fundamentals of Spectrophotometry
Measuring Absorbance
Multiple Choice
In UV-Vis spectrophotometry, what units is absorbance () measured in?
A
Meters ()
B
Moles per liter per centimeter ()
C
It is dimensionless (no units; sometimes reported as absorbance units, AU)
D
Molar ()
Verified step by step guidance1
Understand that absorbance (A) in UV-Vis spectrophotometry is defined by the Beer-Lambert Law, which relates absorbance to concentration and path length.
Recall the Beer-Lambert Law formula: \(A = \varepsilon \times c \times l\), where \(\varepsilon\) is the molar absorptivity (with units typically L mol\(^{-1}\) cm\(^{-1}\)), \(c\) is the concentration (mol L\(^{-1}\)), and \(l\) is the path length (cm).
Note that absorbance is calculated as the logarithm of the ratio of incident light intensity to transmitted light intensity: \(A = -\log_{10} \left( \frac{I}{I_0} \right)\), where \(I_0\) is the incident light intensity and \(I\) is the transmitted light intensity.
Since absorbance is a logarithmic ratio of two intensities (which have the same units), the units cancel out, making absorbance a dimensionless quantity.
Therefore, absorbance is reported without units, although sometimes it is expressed as absorbance units (AU) for convenience, but these are not true physical units.
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