In chemistry, you often need to measure the rate of a reaction. You can accomplish this by placing small amounts of solution from the reactant, at different time points, in a spectrophotometer. The absorbance will change as the rate of reaction changes. If you use a calibration curve of absorbance versus product (which you should have obtained earlier in the experiment) to convert your absorbance to product amount, you can graph the amount of product formed versus time and subsequently find your reaction rate.
Convert your absorbance of solution from the different time points of the experiment to product concentration. Use the equation of your calibration curve, which is a graph of absorbance versus different known concentrations of product. For instance, if your calibration curve states that A=2C, in which A is absorbance and C is concentration, then C=2/A and you can use this fact to convert absorbance to concentration.
Create a graph of time versus product concentration for all of the points you found in Step 1. Graph time on the x-axis and concentration on the y-axis.
Fit a best-fit line, using graphing software, to your time versus concentration curve. Find the slope of the curve, which is the rate at which concentration changes with time. This is the rate of reaction.