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

Solution :
As the concentration of reactant decreases from 0.8 to 0.4 in 15 minutes hence the $${t_{\frac{1}{2}}}$$ is 15 minutes. To fall the concentration from 0.1 to 0.025 we need two half lives i.e., 30 minutes.