SOLUTION: strontium-89 has a half-life of 50.6 days. How long will it take for 20 grams of strontium-89 to decay to 8 grams?

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Question 372366: strontium-89 has a half-life of 50.6 days. How long will it take for 20 grams of strontium-89 to decay to 8 grams?
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
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strontium-89 has a half-life of 50.6 days.
How long will it take for 20 grams of strontium-89 to decay to 8 grams?
:
The half life formula: A = Ao*2^(-t/h)
where
A = resulting amt after t time
Ao = initial amt
t = time
h = half-life of substance
:
20*2^(-t/50.6) = 8
:
2^(-t/50.6) = 8%2F20
:
2^(-t/50.6) = .4
:
Using nat logs
-t%2F50.6ln(2) = ln(.4)
-t%2F50.6 = ln%28.4%29%2Fln%282%29
-t%2F50.6 = -1.3219
t = -50.6 * -1.3219
t ~ 66.9 days