Question

The half life of a radioactive material is 100 s. Then after 5 min., in 8 gm material remaining active material is?

• A. 1 gm
• B. 4 gm
• C. 2 gm
• D. 1.5 gm

Answer 1

The correct option is A 1 gm

Time $t=5\times 60s=300s$

So number of half lives, $n=\frac{t}{t_{1/2}}=\frac{300}{100}=3$

Using the following relation between remaining amount $N$ and initial amount $N_0$,

$N=N_0(\frac{1}{2}{)}^n$,

we get $N=\frac{N_0}{8}$, where $N_0=8gm$

so after 5min the remaining amount, $N=1gm$