the first order reaction is when the rate of the reaction is directly proportional to the concentrations of one of the reactants. if the reactant whose concentration determines the rate - N rate equation is rate = k [N]¹ where k - rate constant the differential equation is as follows; [N] = [N₀] e^ (-kt) where N - concentration after t minutes N₀ - concentration when t = 0 minutes k -rate constant t - time taken in minutes substituting the values given following equation is obtained 0.088 m = 0.13 e^ (-0.33 *t) e^ (-0.33 *t) = 0.088/ 0.13 -0.33t = ln 0.676 - 0.33t = -0.39 t = 1.18 minutes it takes 1.18 minutes for reactant concentration to decrease from 0.13 m to 0.088 m