Posted by **devin** on Monday, December 5, 2011 at 7:38pm.

If the radiicativr decay of nuclear explosion contains 90 Sr , with a half life of 28.1 years how long will it take for 99 of this radioaioot to decay

- chemistry -
**DrBob222**, Monday, December 5, 2011 at 7:50pm
99 what? 99 grams? 99 tons? 99%. I suspect 99%.

Two steps. First determine k.

k = 0.693/t_{1/2} and substitute into the following:

ln(No/N) = kt

I would set No = 100 if that is 99%.

Then N = 1% (if 99% is gone)

k from above

Solve for t in years.

- chemistry -
**devin**, Monday, December 5, 2011 at 8:01pm
Sorry don't understand it is 99 percent

- chemistry -
**devin**, Monday, December 5, 2011 at 8:09pm
Ln(100)/1)=(0.693/t^1/2)(t). ??? Don't get it

- chemistry -
**DrBob222**, Monday, December 5, 2011 at 10:58pm
There isn't anything to get. Can you punch the calculator? That's all there is to it. Do it in steps. Don't try to muddle it up by putting all of it together.

ln 100/1 = 4.605

0.693/28.1 = 0.02466 = k

so

ln(100/1) = 0.02466*t

4.605 = 0.02466t

and t = 4.605/0.02466 = ? or about 200 years more or less.

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