Posted by Jazmin on .
K42 has a halflife of 12.4 hours. If you begin with 250g of
K42, determine how much you will have left after 4 halflives.
How long does 4 halflives take?
please help

Chemistry 
DrBob222,
There is a shorter way, I think, but this is what I would do.
k = 0.693/t_{1/2}
Substitute k into the equation below.
ln(No/N) = kt
No = 250
N = unknown
k from above.
t = time = 4*12.4 = ??
The short way is (n = number of half lives)
1/2^n = fraction left
1/2^4 = ?? and ?? x 250 = xx.
You should get the same answer either way.