SOLUTION: The half-life of a substance is the time it takes for half of the substance to remain after natural decay. Radioactive water (tritium) has a half-life of 12.6 years. How long will

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Question 298119: The half-life of a substance is the time it takes for half of the substance to remain after natural decay. Radioactive water (tritium) has a half-life of 12.6 years. How long will it take for 85% of a sample to decay?
Answer by ankor@dixie-net.com(22740) (Show Source):
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The half-life of a substance is the time it takes for half of the substance to remain after natural decay.
Radioactive water (tritium) has a half-life of 12.6 years.
How long will it take for 85% of a sample to decay?
:
The half-life formula: A = Ao*2^(-t/h)
Where
A = resulting amt after t time
Ao = initial amt
h = half-life of the substance
t = time
:
If we start with 1 unit, after 85% has decayed we will have .15 units left (A)
:
1*2^(-t/12.6) = .15
using nat log
ln(2^(-t/12.6)) = ln(.15)
log equiv of exponents
*ln(2) = ln(.15)
:
*.693 = =1.897
:
=
:
= -2.737
Multiply both sides by -12
t = -2.737 * -12
t = 34.5 years for 85 % to decay
:
:
Check on your calc: enter: 2^(-34.5/12.6) should =.14988 ~ .15