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

Given that the concentration of the reactant decreases from 0.8 M to 0.4 M in 15 minutes.t1/2 = 15 minute.So, the concentration of reactant will fall from 0.1 M to 0.025 M in two half lives.i.e., 2t1/2 = 2 ×15 = 30 minutes.