chemistry
posted by devin .
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

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. 
Sorry don't understand it is 99 percent

Ln(100)/1)=(0.693/t^1/2)(t). ??? Don't get it

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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