Here rate of disintegration requiredR = 8.0 mCi
= 8.0 × 10–3 × 3.7 × 1010diss–1 = 29.6 × 107diss–1
Half life T1∕2 = 5.3 years = 5.3 × 3.16 × 107 s
But R = λN =

0.693

T1∕2

. N
No. of atoms for given rate required,
N =

RT1∕2

0.693

= = 7.15 × 1016 atoms
As 1 mole i.e., 60 g of cobalt contains 6.023 × 1023 atoms, so, the mass of cobalt required for given rate of disintegration
=

60×7.15×1016

6.023×1023

= 7.123 × 10−6 g