Question

Two samples of radioactive substances initially contain equal number of atoms. Their half life times are respectively $2$ hours and $4$ hours. The ratio of their disintegration rates after $12$ hours is :

• A. $1:4$
• B. $1:8$
• C. $3:4$
• D. $2:3$

Answer 1

The correct option is B $1:8$

In one half life the activity or disintegration rate is halved. After 12 hours first sample would have six half lives and the second sample would have 3 half lives. Ratio of disintegration would be $\frac{1}{2^6}:\frac{1}{2^3}$ which is $1:8$.