Question

In the first order reaction, the concentration of the reactants is reduced to $25$% in one hour. The half life period of the reaction is:

• A. $2$ hrs
• B. $4$ hrs
• C. $1/2$ hr
• D. $1/4$ hr

Answer 1

The correct option is C $1/2$ hr

According to the theory of chemical reactions, in a first order reaction, the concentration of the reactant present falls by $50\%$ after every half-life.

Considering that the concentration given above is $25\%$, we can look at it in the following way:
After the first half-life, concentration becomes $50\%$.
After the next half-life, it becomes half of $50\%$ i.e., $25\%$.

Hence, in the above question, 2 half-lives have been completed. Since 2 half-lives make an hour, the half-life period of the above reaction is 30 minutes.