Posted by **matt** on Monday, September 10, 2007 at 8:43pm.

suppose you have 100grams of radioactive plutonium-239 with a half-life of 24,000 years. how many grams of plutonium-239 will remain after:

A)12,000 years

B)24,000 years

C)96,000 years

how do i do this??????

- science -
**DrBob222**, Monday, September 10, 2007 at 8:48pm
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**, Monday, September 10, 2007 at 9:17pm
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...**, Monday, September 10, 2007 at 8:50pm
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**, Monday, September 10, 2007 at 9:15pm
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.)

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