Question

In a mean life of a radioactive sample.

• A. About 1/3 of substance disintegrates
• B. About 2/3 of the substance disintegrates
• C. About 90% of the substance disintegrates
• D. Almost all the substance disintegrates

Answer 1

Answer: (B)

About 2/3 of the substance disintegrates

Answer 2

By using $N = N_{0}e^{- \lambda t}$ and $t = \tau = \frac{1}{\lambda}$

Substance remains $= N = \frac{N_{0}}{e} = 0.37N_{0}\widetilde{–}\frac{N_{0}}{3}$

∴ Substance disintegrated $= N_{0} - \frac{N_{0}}{3} = \frac{2N_{0}}{3}$.