Question

How does measuring absorbance allow us to measure a rate law?

Answer 1

Reaction rates can be determined experimentally by measuring the concentration of a reactant and/or product over time.

Physical measurements can be performed on a system while it is reacting. These measurements have the advantage of not disturbing the system, and they can be taken during the reaction.

Beer's law can be employed in following either formation or decay of a species during the reaction by measuring its absrobance wih respect to time if the species being formed or being consumed has an absorbance spectrum different from the other species in solution.

So absorbance can be used to measure rate