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

Options

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

Answer

A

Explanation

Here, t=2 `tau=2xx1.44 T_(1//2)=2.88 T_(1//2)`
As `N/N_0=m/m_0=(1/2)^(t//T_(1//2))=(1/2)^(2.88)`
`therefore m=m_0(1/2)^(2.88)=10(1/2)^2.88 ` g
=10 x 0.1358 g = 1.358 g