Gather information from your pre-lab and lab notes. Write out the stoichiometric equation for the reaction, such as A + 2 B ' 3 C. Record the change in concentration of each reactant and product. Write down the amount of time during which those changes occurred.
Write out the equation for the rate of reaction, r = change in concentration divided by change in time. Select one reactant or product. Multiply the equation by -1 if you selected a reactant to ensure you get a positive rate of reaction. Multiply the equation by 1 divided by the stoichiometric coefficient of the species you chose. For example, r = - (1/2) (change [B] / change in time). In this equation, two B combine with one A to make three C.
Repeat your calculation with another species. For example, r = (1/3) (change [C] / change in time). Record the product as the rate of reaction if both equations provide the same value for r. Repeat your calculation with a third species if the equations provided different values for r. Record the average of all r-values as the rate of reaction.