Chemistry
posted by Mindy .
The halflife for beta decay of strontium90 is 28.8 years. A milk sample is found to contain 10.3 ppm strontium90. How many years would pass before the strontium90 concentration would drop to 1.0 ppm?

k = 0.693/t_{1/2}
Substitute k into the equation below.
ln(No/N) = kt
No = 10.3 ppm
N = 1.0 ppm
k from above.
t = ? 
DR.BOB222 ENGLISH PLEASE?????? I know how to get the beta decay for strontium. just having a hard time with the concentration of strontium and the amount of yrs. passed....please help!!

All you need to do is to substitute 28.8 years into equation 1 I gave you to solve for k, then substitute k into the
ln(No/N) = kt and solve for t.
I TOLD you what to substitute for No and I told you what to substitute for N and k. You have only to solve for t. t is approximately 100 years for the Sr to decay to about 1.0 ppm. 
the halflife of cobalt60 is 5.20 years.. how many milligrams of a 2.000mg sample remains after 6.55 years?