Question
The activity of a freshly prepared radioactive sample is 10^10 disintegrations per second, whose mean life is 10^9 sec.
The mass of an atom of this radioisotope is 10 ^ -25 kg. The mass (in mg) of the radioactive sample is
Joshi sir comment
dN/dt = -λN
1010 = -[1/109] N λ = 1/mean life
so N = 1019 disintegration
so mass of radioisotope = 1019 *10-25 = 10-6 Kg = 1 mg