CAPP Chemistry Lab Practice Test 2025 – Complete Exam Prep

To determine the concentration of an unknown solution using spectroscopy, it's essential to measure the absorbance of the solution and apply the Beer-Lambert Law. This law establishes a direct relationship between the absorbance of light by a solution and its concentration, along with the path length of the light through the solution and the molar absorptivity, which is a characteristic of the compound being measured.

When light passes through a solution, some of the light is absorbed by the solute, and the amount of light absorbed (which is measured as absorbance) can be quantitatively related to the concentration using the equation:

\[ A = \epsilon c l \]

In this equation:

- \( A \) is the absorbance (a unitless measure),

- \( \epsilon \) is the molar absorptivity (in L/(mol·cm)),

- \( c \) is the concentration of the solution (in mol/L), and

- \( l \) is the path length of the light through the solution (in cm).

By measuring the absorbance at a specific wavelength where the solute absorbs light and knowing the molar absorptivity and the path length, one can rearrange the equation to solve for the concentration of the unknown