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

The correct option is D 1.35 gm

By using $M=M_0{e}^{-\lambda t}\Rightarrow M=10e^{-\lambda \left(2\tau \right)}=10e^{-\lambda \left(\frac{2}{\lambda }\right)}=10{\left(\frac{1}{e}\right)}^2=1.359gm$