Question

In a first order reaction, the concentration of the reactant, decreases from 0.8M to 0.4M in 15minutes. The time taken for the concentration to change from 0.1M to 0.025M is

(a) 30 minutes 

(b) 15 minutes

(c) 7.5 minutes 

(d) 60 minutes.

Answer 1

(a) : The concentration of the reactant decreases from 0.8M to 0.4M in 15 minutes,
i.e. t_1/2 = 15 minute.
Therefore, the concentration of reactant will fall from 0.1M to 0.025M in two half lives.
i.e. 2t_1/2 = 2 × 15 = 30 minutes.