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

Options

(a) 2.50 gram
(b) 3.70 gram
(c) 6.30 gram
(d) 1.35 gram

Correct Answer:

1.35 gram

Explanation:

M = M₀ e_-λt
Since, t = 2/λ, we get
M = 10 × e_{-λ × (2/λ)} = 10 e⁻² = 1.35 gram