Question

In a first order reaction the concentration of reactant decreases from $0.6\text{M}$ to $0.3\text{M}$ in
$30\text{min}$. The time taken for the concentration to change from $0.1\text{M}$ to $0.025\text{M}$:

• A. $60\text{min}$
• B. $30\text{min}$
• C. $15\text{min}$
• D. $50\text{min}$

Answer 1

The correct option is A $60\text{min}$

Time taken to reduce reactant from $0.6\text{M}$ to $0.3\text{M}$ means $t_{1/2}=30\text{min}$.
From $0.1\text{M}$ to $0.025\text{M}$, Concentration has become $\frac{1}{4}$ times.That is reactant is reduced by $\frac{1}{4}$ times of the initial.
$t_{3/4}=2t_{1/2}=2\times 30=60\text{min.}$
($\therefore$$t_{1/2}$ value is independent of initial concentration for a first order reaction).