A sample of

Question

131
I was purified and then analyzed for iodine content. It was found that the
sample was 99.8% pure. 25 hours later the sample was tested again and it was found that the
sample was now just 78.2% pure. What is the half
-
life of
131
I?

Answer 1

ln(No/N) = kt
No = 99.8
N = 78.2
k = solve for this
t = 25 hours

Then k = 0.693/t_1/2
Solve for t_1/2
You probably do better to write I-131.

Answer 2

Half-life of I-131 is 71.03hours