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September 30, 2014

September 30, 2014

Posted by **Kristina** on Thursday, July 21, 2011 at 9:55pm.

a) How long will it take for 10% of the U-238 atoms in a sample of U- 238 to decay?

b)If a sample of U-238 initially contained 1.9×1018 atoms when the universe was formed 13.8 billion years ago, how many U-238 atoms will it contain today?

- Chem 2 -
**DrBob222**, Thursday, July 21, 2011 at 10:04pmk = 0.693/t

_{1/2}

Then

ln(No/N) = kt

I would call No = 100

Then N = 90(after 10 have decayed--which is 10%)

k from above. Solve for t in years.

b.

Same procedure. No = atoms you started with. N = atoms remaining.

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