Posted by matt on .
suppose you have 100grams of radioactive plutonium239 with a halflife of 24,000 years. how many grams of plutonium239 will remain after:
A)12,000 years
B)24,000 years
C)96,000 years
how do i do this??????

science 
DrBob222,
Here is how you do the first one.
k = 0.693/t_{1/2}
Substitute 24,000 for t_{1/2} in the above equation and calculate k. Then substitute k into the following equation.
ln(No/N) = kt
No = 100 grams = what you start with.
N = what you end up with and this is the unknown.
k from above.
Solve for N.
Use the same procedure for parts B and C. The answer for N will be in grams since No is in grams.
Post your work if you get stuck. 
science 
DrBob222,
t in the second equation is, of course, the time which for part a is 12,000, part b is 24,000, etc.

science 
Food4thought...,
Well, there would be 50 grams left after 24,000 years. And if you divide both by two, you get __grams(try that yourself!) after 12,000 years. And, if you multiply that (1st problem I did) by 2, you get __grams after 96,000 years. Notice that everything is a multiple of 2.
Hope that helps!
Let me know what answers you got for the A and C (I gave you the answer for B), and I'll tell you if you're right! :) 
science 
DrBob222,
Food4thought needs to rethink the answers s/he gave. The answer for 24,000 years (part B) is correct. I don't understand the answer for part A (12,000 years) and the answer for part C gives the same amount (2 x 50 = 100 g) we started with so it CAN'T be correct. Not much truth and little reality in these answers. (Sorry, I couldn't resist.)