Question

In 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. $30minutes$
• B. $60minutes$
• C. $7.5minutes$
• D. $15minutes$

Answer 1

The correct option is A $30minutes$

The concentration of the reactant, decreases from 0.8 M to 0.4 M in 15 minutes. It is equal to the half life period.
The time taken for the concentration to change from 0.1 M to 0.025 M is equal to two half lives, i.e., $2\times 15=30minutes.$
This is because when the concentration changes from 0.1 M to 0.025 M, it is reduced to one fourth.