Question

A solution of a compound in ethanol shows an absorbance of 0.52 at 236 nm in a cell with a 1 cm light path. Its molar absorptivity in ethanol at that wavelength is 12,600 M^-1 cm^-1. What is the concentration of the compound?

Short answer

The concentration (c) of the unknown compound is 4.1x10^-5M.

Step-by-step solution

Beer-Lambert Law

The Beer-Lambert law is the relationship between the attenuation of light through a substance and the properties of that substance.

Beer-Lambert law is used in chemistry to measure the concentration of chemical solutions, analyze oxidation, and measure polymer degradation. This law fails at high concentrations due to electrostatic interactions between molecules in proximity. The Beer-Lambert law is mathematically presented as, where A is absorbance, is molar absorptivity, l is the length of the path of light, and c is concentration[l1] .

[l1]Concentration of what?

Determining concentration of compound in ethanol

It is given that a solution of a compound in ethanol shows an absorbance (A) of 0.52 at 236 nm in a cell with a 1 cm light path and its molar absorptivity () in ethanol at that wavelength is 12,600 M^-1cm^-1.

By applying the Beer-Lambert law, where A = c l, the concentration of the unknown compound can be calculated as shown below.

$\text{C=}\frac{\text{A}}{/\epsilon }$

$$\begin{gathered} \text{c}=\frac{0.52}{1\times 12600} \\ \text{c}=4.1\times {10}^{-5}\text{M} \end{gathered}$$

Hence, the concentration (c) of the unknown compound is 4.1x10^-5M.