Question

The half-life of 232-Th is 1.4 x 10^10 years.if there were 25.0 grams of thr sample left after 2.8 x 10^10 years,how many grams were in the orifinal sample

Answer 1

1.    For this problem, we use the formula for radioactive decay which is expressed as follows:

An = Aoe^-kt

where An is the amount left after time t, Ao is the initial amount and k is a constant. 

We calculate as follows:

An = Aoe^-kt
0.5 = e^-k(1.4x10^10)
k = 4.95x10^-11

An = Aoe^-kt
25 = Aoe^-4.95x10^-11(2.8x10^10)
Ao = 100 g