Question

In a first order reaction, the concentration of the reactant is reduced to $12.5\%$ in one hour. When was it half completed?

• A. 3 hr
• B. 20 min
• C. 30 min
• D. 15 min

Answer 1

The correct option is B

20 min

Number of half lives $=100\stackrel{{}_{\frac{1}{2}}^t}{\to }50\stackrel{{}_{\frac{1}{2}}^t}{\to }25\stackrel{{}_{\frac{1}{2}}^t}{\to }12.5=3$.

For 3 half lives = 1 hour = 60 min

1 half life = 20 min