A.P. Chemistry
posted by John .
Thorium232 undergoes alpha decay and has a halflife of 1.4 * 10^10 years. How long will it take for a 1.00mg sample of thorium232 to be reduced to 0.5mg?

The simple answer:
It will take 1.4E10 years.
If the half life is 1.4E10 and you want exactly 1/2 of the Th to decay, it will take just one half life. right?
You can work it out.
k = 0.693/t_{1/2}
ln(No/N) = kt.
No = 1.0 mg
N = 0.5 mg
k from above.
Solve for t.
You should obtain 1.4E10 years.