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 H₂O₂
                                                      
H₂O₂    H₂O + ½ O₂

If [A₀] = initial conc. At t = 0 and [A] is conc. At time t then the conc. Of H₂O₂ at different intervals can be calculated by titration of the reactant using KMnO₄ in acidic medium. At any time volume of KMnO₄ ≡ [H₂O₂]₄ at time t.

Thus, V₀ = volume of KMnO₄ at start = [A₀]

V_t = volume of KMnO₄ at time t = [A]

Hence for first order reaction, k = 2.303/t log V₀/V_t.

(ii) Decomposition of nitrogen pentaoxide

N₂O₅ 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, N₂O₄ and NO₂ remain in solution and the volume of oxygen gas collected is noted at different intervals of time. Thus,

Volume of O₂ gas collected at any time (V_t) ∝ Amt. of N₂O₅ decomposed (x)
                                                              
i.e. x ∝ V_t

Also vol. of O₂ gas collected at infinite time (V∞) Amt. of N₂O₅ taken initially (a)
                                                               
i.e. a ∝ V∞

Using the first order equation
                                            
k = 2.303/t log V∞/(V∞ - V_t)