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

Options

A. `6.30` gm
B. `1.35` gm
C. `2.50` gm
D. `3.70` gm

Answer

B

Explanation

Using the relation for mean life .
Given : `t = 2 tau = 2 ((1)/(lambda)) ( therefore tau = (1)/(lambda))`
Then from `M = M_(o) e^(-lambdat) = 10 e^(- lambda xx (2)/(lambda)) = 10 ((1)/(e))^(2) = 1.35` g