SOLUTION: Radioactive Decay. Strontium-90 has a half-life of 28 years. How long will it take for 40 grams of this to decay to 5 grams? Apply the formula N= N0e^rt, where N is the number of g

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Question 1126133: Radioactive Decay. Strontium-90 has a half-life of 28 years. How long will it take for 40 grams of this to decay to 5 grams? Apply the formula N= N0e^rt, where N is the number of grams.

Round your answer to the nearest year.
Found 4 solutions by solver91311, ikleyn, josgarithmetic, Alan3354:
Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!


Physics problem. What is r for a half-life of 28 years? Once you know that, plug in your numbers and solve for .


John

My calculator said it, I believe it, that settles it

Answer by ikleyn(52814) About Me  (Show Source):
You can put this solution on YOUR website!
.
The ratio  of 5 grams to 40 grams is  5%2F40 = 1%2F8, which is  (fortunately !) exactly  %281%2F2%29%5E3.


It means that 40 grams will decay to 5 grams in the time which is exactly thrice the half-period, or exactly in 3*28 = 84 years.

Solved.

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The instruction placed at the end of your post is VERY BAD advise on how to solve the given problem.

The much simpler way is that I showed in my post.


Answer by josgarithmetic(39620) About Me  (Show Source):
You can put this solution on YOUR website!
Half-life of 28 years given


1%2Ae%5E%28rt%29=1%2F2
e%5E%28rt%29=1%2F2
ln%28e%5E%28rt%29%29=ln%281%2F2%29
r%2At=ln%281%2F2%29

Your already know that for this, t=28.
r=ln%281%2F2%29%2F28


You should be able to use this for answering the question and using the equation in your given instructions.

Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
Radioactive Decay. Strontium-90 has a half-life of 28 years. How long will it take for 40 grams of this to decay to 5 grams? Apply the formula N= N0e^rt, where N is the number of grams.
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Years    Remaining
 0         40
 1         20
 2         10
 3          5