Question

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

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

Answer 1

Answer: (A)

1.35 g

Answer 2

$N = N _{0} e ^{-\lambda t }$
$\Rightarrow m = m _{0} e ^{-\lambda t }$
$= m _{0} e ^{-\lambda(\frac {2}{\lambda})}$
$=\frac{10}{ e ^{2}}$
$=1.35\, gm$

So, the correct option is (A): $1.35\ gm$