How many half-lives will pass by the time 1.56% of I-131 is present? B. Approximately how many days does that equal? *

Answer 1

There are a couple of equations one could use for this type of problem, but I find the following to be the easiest to use and to understand.

Fraction remaining (FR) = 0.5ⁿ

n = number of half lives that have elapsed

In this problem, we need to find n and are given the FR, which is 1.56% or 0.0156 (as a fraction).

0.0156 = 0.5ⁿ

log 0.0156 = n log 0.5

-1.81 = -0.301 n

n = 6.0 half lives have elapsed