I have 2 two problems realated to a radioactive exercise.

^{228}Th is a member of the thorium series (ending with the stable

^{208}Pb). What volume in cm

^{3} of helium at 0

^{0}C and 1atm collected when 1.00 gram of

^{228}Th (t

_{1/2}=1.91 years) is stored in a cointainer for 20.0 years. The half-lives of all intermadiaste nuclides are short compared to the half-life of

^{228}Th.

The first solution that I've made is calculating:

-the mole of

^{228}Th remained after 20 years

-the mole of

^{228}Th has decayed

-the mole of Helium based on the equation:

^{228}Th

^{208}Pb+5

^{4}He

-the volume of helium

and the result is 0,49 L

But the result is 3.56 L. The key is calculating the activity of Thorium, then they have the number of Helium and finally have the volume of Helium

So what is the difference between two solutions?