First slide

Radio activity

Question

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

Moderate

A

2.50 gm
B

3.70 gm
C

6.30 gm
D

1.35 gm

Solution

Mean life $T_{m} = \frac{1}{λ}$
We know, $N = N_{0} e^{- λt} = 10 x e^{- λ (2 x \frac{1}{λ})} = \frac{10}{e^{2}} = \frac{10}{{(2.7)}^{2}} {gm}^{2} = 1.35 gm$

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