CHEM: iodine?

Question

The half-life of iodine-123 is 13.2 hours. What percentage of iodine-123 remains in a sample after 36 hours?

a.95%
b.37%
c.85%
d.15%
e.13%

Answer 1

13.2 x 3600 = 47520 s
36 x 3600 = 129600 s
129600 / 47520 = 2.73
1 / 2.73 x 100 = 37%

Answer 2

Use ln 2 = k x t(1/2) to find the rate constant for decay. Then use

ln (A/A0) = -k t

Alternatively, note that 36 hr is around 2 3/4 half lives, and after three half lives, you would have just 12 1/2% remaining (why?). You can now pick out the correct answer. It is NOT 37%.