Iodine-131 has a half life of 2.2 × 10-2 year. How long will it take to reduce a 130 g sample of 131I to 74 g 131I?

Answer 1

k = 0.693/t_1/2
ln(No/N) = kt
No = 130 g
N = 74 g
k = from above
Solve for t in years.