Question

How does absorbance in a UV/vis experiment vary with concentration?

Answer 1

Absorbance varies linearly with concentration. The absorbance value at a given frequency (i.e. colour) corresponds to solution concentration.

Explanation:

Typically, an analyst prepares various solutions of a transition metal ion at different concentrations, and measures their absorbance value by UV/Vis spectrophotometry. A calibration curve is constructed as the analysis can see how well absorbance varies with concentration. The analyst then measures the absorbance value of an unknown, and estimates the concentration of the unknown by interpolating the absorbance value on the calibration curve. See here.

Here is a video showing an experiment to determine the concentration of an unknown sample of iron (III) chloride.