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

Options

A. 5g
B. 2.7g
C. 1.35g
D. 0.8g

Answer

C

Explanation

Mean life `tau = 1/lamda` and time `t=2tau = 2/lamda " and " M_0 = 10g`
`therefore` mass of the element left after time t is given by
`M=M_0 e^(-lamdat)` (Decay law)
`=10e^(-lamdaxx2/lamda) = 10e^(-2) = (10)/(e^2) = (10)/((2.71)^2) = 1.35 g`