Question

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 -

• A. 30 minutes
• B. 15 minutes
• C. 7.5 minutes
• D. 60 minutes

Answer 1

Answer: (A)

30 minutes

Answer 2

1M $\overset{\quad t_{1/2}\quad}{\rightarrow}$0.05M $\overset{\quad t_{1/2}\quad}{\rightarrow}$0.025M

t = 2 × t_1/2 = 2 × 15 = 30 minute