Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 6

Big Idea #4: Chemical Reactions and their Rates

BEER’S LAW

To measure the concentration of a solution over time, a device called a spectrophotometer can be used in some situations. A spectrophotometer measures the amount of light at a given wavelength that is absorbed by a solution. If a solution changes color as the reaction progresses, the amount of light that is absorbed will change. Absorbance can be calculated using Beer’s Law:

Beer’s Law

A = abc

A = absorbance

a = molar absorptivity, a constant that depends on the solution

b = path length, the distance the light is traveling through the solution

c = concentration of the solution

As molar absorptivity and path length are constants when using a spectrophotometer, Beer’s Law is often interpreted as a direct relationship between absorbance and the concentration of the solution. Beer’s Law is most effective with solutions that visibly change color over the course of a reaction, but if a spectrophotometer that emits light in the ultraviolet region is used, Beer’s Law can be used to determine the concentrations of reactants in solutions that are invisible to the human eye.

You may also run into a device called a colorimeter while studying Beer’s Law. A colorimeter is simply a spectrophotometer that can only emit light at specific frequencies, whereas a spectrophotometer can emit light at any frequency within a set range.