There are 5g of 131I left after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days?

Question

What is the half life of a radioisotope if you started with 1 gram and 0.250 grams is left after 6 days?

im not looking for a free answer, I've tried working it out with what the teacher gave us and cannot figure these out, thank you :)

Answer 1

There are 5g of 131I left after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days?
Look up the half life of I-131 (that may have been given in the problem) and use it to calculate k for the decay. k = 0.693/t1/2
Then ln(No/N) = kt
You know N, k and t, solve for No.

What is the half life of a radioisotope if you started with 1 gram and 0.250 grams is left after 6 days?
ln(No/N) = kt. You know t, No = 1 grams; N = 0.250 g. Solve for k, then
k = 0.693/t1/2. Knowing k you can calculate t1/2.
Post your work if you get stuck.