Question

A sample of a radioactive element has a mass of $10g$ at an instant $t=0$. The approximate mass of this element in the sample after two mean lives is

• A. $1.35g$
• B. $2.50g$
• C. $3.70g$
• D. $6.30g$

Answer 1

The correct option is A $1.35g$

Mean Life $=\frac{1}{\lambda }$
$N_o=10gms$
First order decay:
$N=N_o{e}^{-\lambda t}$
At $t=\frac{2}{\lambda }$
$N=N_o{e}^{-2}=1.35gms$