What is half-life of a first order reaction if time required to decrease concentration of reactant from $0.8…
What is half-life of a first order reaction if time required to decrease concentration of reactant from $0.8 \mathrm{~mol} \mathrm{dm}^{-3}$ to $0.2 \mathrm{~mol} \mathrm{dm}^{-3}$ is 12 hour?
6 hour
3 hour
1.5 hour
12 hour
Solution
If conc. decreases from $0.8 \frac{\mathrm{mol}}{\mathrm{dm}^3}$ to $0.2 \frac{\mathrm{mol}}{\mathrm{dm}^3}$ it means $75 \%$ of reactant is consumed
$\begin{aligned}
& \therefore \mathrm{t}_{75 \%}=2 \times \mathrm{t}_{1 / 2} \\
& \text { or } \mathrm{t}_{1 / 2}=6 \mathrm{hr}
\end{aligned}$