Question

a radioactive substance (M= 100g/mol) have t½ of 5 days. if today 0.125mg is left over what was its activity 20 days earlier?

(a) 522.8 Ci

(b) 1.93*10^7 Rd

(c) 1.93*10^13 Bq

(d) all of these

Joshi sir comment

20 day means 4 half lifes so today 0.125mg means 0.125*24 = 2gm earlier 

so activity dn/dt = kn

solve and convert in required units

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