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Nuclear Chemistry And Half-Life

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sweetdaisy186:
Hey guys!

I am having a very stuff time understand nuclear chemistry and half-life. The problem is:

The half-life of iodine-131 is 8.04 days. Suppose we follow the activity of a sample of iodine-131 while it falls to 10% of its inital value. (a) Estimate how long this will take, and (b) calculate a more exact value of the time required

When they say that the half-life is 8.04 days it means that is how long before half of the iodine 131 disappears right? So, should I take 10 percent of 8.04 for parts and b?

Any hints would be greatly appreciated!!!

Thanks!

Donaldson Tan:
you may refer to this similar discussion thread here:
http://www.chemicalforums.com/index.php?board=27;action=display;threadid=4094

yl88:
Since this is a first order rxn, the integrated rate law gives ln(Nt/N0)=-kt
k=ln2/half life=0.08621
Nt/N0=10%=0.1
ln0.1=-0.08621t
t=26.70830188days

how do you estimate the time without any calculation?

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