Posted by **Noah** on Sunday, February 19, 2012 at 5:08pm.

Uranium-238 decays to Lead-206. The half life of Uranium-238 is 4.5 billion years. If you have a 100 g sample. How much sample will you have after 2 half lifes?

- Half-Life -
**Reiny**, Sunday, February 19, 2012 at 5:18pm
just do it in your head ...

after 1 half-life you would have 50 g left

after 2 half-lifes you would have 25 g left

- Half-Life -
**DrBob222**, Sunday, February 19, 2012 at 6:13pm
Your question isn't quite clear to me. If you want the mass of U after two half lives, then 25 g is correct. If you want the mass of sample left after two half lives, that is 25g U + the equivalent of 75 g U that's been converted to Pb. That conversion is 75 g U x (atomic mass Pb/atomic mass U) = about 75*207/238 = about 65 g. Therefore, the total mass is 25 + about 65 = about 90 g.

- Half-Life -
**Dee**, Friday, March 22, 2013 at 11:11pm
If the half-life of a radionuclide is 1 month, is a sample of it completely decayed after 2 months?

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