Question

Half-life period for a first order reaction is 10 s. The time required to reduce 2.0 g of the reactant to 0.25 g is:

• A. 20 s
• B. 50 s
• C. 30 s
• D. 15 s

Answer 1

The correct option is C 30 s

As amount of substance left $N=\frac{N_0}{2^n}$ where n is the number of half-lives, $N_0$ is the initial amount.
$\text{As}0.25=\frac{2}{2^n}$
$\text{Or}{2}^n=\frac{2}{0.25}=8$
$\Rightarrow n=3$
$\therefore \text{time required in the reduction}=3\times 10=30\text{s}$