alleryStart learning →Iodine-131 decays by a first order process with a rate constant of 0.138 per day. What is its half-life?
A5.02 days
B7.24 days
C13.8 days
D16.7 days
Answer & Solution
Correct answer: A. 5.02 days
1. Radioactive decay is first order, so t(1/2) = 0.693/k applies.
2. Substituting gives t(1/2) = 0.693 divided by 0.138 per day.
3. That is 5.02 days.
4. The value 7.24 days is 1/0.138, which forgets the factor of ln 2.
5. The value 16.7 days is the time for 90 percent decay, not for half.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 12 "Kinetics", section 12.4 Integrated Rate Laws_
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