Iodine-131 has a half-life of 8 days. How many grams of a 12 g sample will remain at the end of 40 days?

Question

how do i solve this? i thought it was
12^40=1.5 but that's wrong...

Answer 1

Two ways to do it but both actually are the same.
k=0.693/t_1/2
Substitute k into the equation below.
ln(12/N) = kt
Solve for N. I get 0.375 g.

Second way:
1/(2ⁿ)* 12
[1/2^(40/8)]*12 = ??
Again, 0.375g.