Question

If half life of a first order reaction is 10 minutes, find the time required to decrease concentration of reactant from $0.08 \text{ M}$ to $0.02 \text{ M}$.

• A. 10 minutes
• B. 20 minutes
• C. 30 minutes
• D. 40 minutes

Hint:

First order: Every half-life halves concentration

Answer 1

The Correct Option is B

Concept: For first order reaction:
• Half-life is constant.
• Each half-life reduces concentration to half.

Step 1: Track concentration change. \[ 0.08 \rightarrow 0.04 \rightarrow 0.02 \]

Step 2: Count number of half-lives.
• $0.08 \rightarrow 0.04$ = 1 half-life
• $0.04 \rightarrow 0.02$ = 1 half-life Total = 2 half-lives

Step 3: Calculate time. \[ \text{Time} = 2 \times 10 = 20 \text{ minutes} \]

Step 4: Conclusion.
Thus, required time = 20 minutes. Final Answer: Option (B)