Posted by Sarah H on .
Neptunium239 has a halflife of 2.35 days. How many days must elapse for a sample of 239Np to decay to 1.00% of its original quantity?
I got: close to 15 days. Is this right?
I figured that about 1% of the original quantity would be roughly 6.5 Half lives, and since a half life is 2.35 days, 6.58*2.35

Gen Chem 
DrBob222,
It's done this way for an exact answer.
k = 0.693/t_{1/2}. Substitute k into the following equation and solve for t.
ln(100/1) = kt.
I get 15.6 days.