Determine the reaction rate for one of the substances. Obtain a scatterplot of concentration of reactant versus time. A scatterplot shows two variables, such as concentration and time, on an XY graph as a series of points. Select two adjacent points on the plot, subtract the concentration at the later point from the concentration at the earlier point. Divide the difference by the amount of time between the two points. Multiply the result by -1 to show the rate as a positive value.
Record the above reaction rate. Label with which substance in the ratio it corresponds.
Divide the known reaction rate by the ratio value for that substance. For instance, if the reaction rate of lithium is 2.125 and the relative reactivity of lithium, sodium and potassium (respectively) is 3:5:12, then divide 2.125 by 3.
Multiply the result by the ratio value for the substance for which you want to determine the reaction rate. For example, 2.125 divided by 3 equals .7083. Multiplying the result by 5 provides a reaction rate of 3.542 for sodium.