Potassium-42 has a half-life of 12.4 hours. How much of a 1600g sample of potassium-42 will be left after 36.0 hours?

Answer 1

m = mi e^- k t
1/2 = e^-k *12.4
ln 0.5 = -12.4 k
k = -.693 / -12.4 = .0559
so
m = 1600 e^-.0559*36
= 1600 (.134) = 214
check in head
36 hours is about 3 half lives
so about 1/8 of original
so around 200, ok

Answer 2

1600 * (1/2)^(36.0 / 12.4)