Question

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

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

Answer 1

Answer: (D)

1.35 gm

Answer 2

By using $M = M_{0}e^{- \lambda t}$

⇒ $M = 10e^{- \lambda(2\tau)} = 10e^{- \lambda\left( \frac{2}{\lambda} \right)} = 10\left( \frac{1}{e} \right)^{2} = 1.359$gm