Various methods are employed to calculate the rate constant. The method to be adopted and the proper selection depend on the experimental condition and nature of reactants and products. Some methods employed are discussed below: Volumetric method Used for reactions where reactants or products are acid or base or species undergoes a redox reaction. The idea is illustrated by the example below: (i) Decomposition of H_{2}O_{2} H_{2}O_{2 } H_{2}O + ½ O_{2} If [A_{0}] = initial conc. At t = 0 and [A] is conc. At time t then the conc. Of H_{2}O_{2} at different intervals can be calculated by titration of the reactant using KMnO_{4} in acidic medium. At any time volume of KMnO_{4} ≡ [H_{2}O_{2}]_{4} at time t. Thus, V_{0} = volume of KMnO_{4} at start = [A_{0}] V_{t} = volume of KMnO_{4} at time t = [A] Hence for first order reaction, k = 2.303/t log V_{0}/V_{t}. (ii) Decomposition of nitrogen pentaoxide N_{2}O_{5} decomposes in the gaseous state as well as in the form of its solution in an inert solvent. The decomposition is shown below. When the reaction is dome in its solution, N_{2}O_{4} and NO_{2} remain in solution and the volume of oxygen gas collected is noted at different intervals of time. Thus, Volume of O_{2} gas collected at any time (V_{t}) ∝ Amt. of N_{2}O_{5} decomposed (x) i.e. x ∝ V_{t} Also vol. of O_{2} gas collected at infinite time (V∞) Amt. of N_{2}O_{5} taken initially (a) i.e. a ∝ V∞ Using the first order equation k = 2.303/t log V∞/(V∞ - V_{t})