Choose one of the chemical species to manipulate. For example, given the equation for the decomposition of dinitrogen pentoxide, N2O5(g) -> 2NO2(g) + 1/2 O2(g), choose any chemical compound found in the equation. In this case, N2O5 is the preferential choice because it is isolated on one side of the equation.
Take the inverse of the stoichiometric coefficient of the chosen chemical species. In the example, N2O5 (1 mole of dinitrogen pentoxide) has an understood stoichiometric coefficient of one and therefore the inverse is also one.
Multiply the inverse stoichiometric coefficient by the change in molar concentration of the species per unit of time. This change in molar concentration is determined experimentally during the time of the reaction or, if the equation is well known, can be found in a chemical reaction reference guide. The resulting figure is the rate of reaction in both the forward and the reverse direction. In conclusion, Rate = -("[N2O5]) / ("t) = [(1.7 x 10^-3) * (0.030 M)] / s (where 1.7 x 10^-3 is the change in molarity, 0.030 M is the initial molarity and s is unit seconds) yields a final answer of 5.1 x 10^-5 M/s.