A sample of radioactive element has a mass of 10g at an instant t=0. The mass ( in gm) of the element in the sample after 2-mean lives is ___ x 10^-2 gm. $(\frac{1}{e^{2}} = 0.135)$

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answer is 135.

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Detailed Solution

$\begin{array}{l} M = M_{0} e^{- λt} \\ ∴ t = 2 t_{m} \Rightarrow t = \frac{2}{λ} \\ M = M_{0} e^{- 2} \\ = 10 (0 .135) \\ = 1 .35 gms \\ = 135 \times 10^{- 2} gm \end{array}$

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