Half life

Resolved Question

Half life question...?
Hi, can someone give me a hand with this, i think i got the right idea, but it don't seem to give me the answer provided.

The radioactive isotope 51Cr is used to evaluate the lifetime of red blood cells.Its t½ = 27.7 days. A hospital laboratory has 80.0 mg of 51Cr. How much of 51Cr is left after 85 days?
Answer given = 9.6mg

by joeyeehu...
Best Answer - Chosen by Asker

All radioactive decay is a first order reaction.

First, you have to calculate the rate constant, k = 0.693/ t1/2

Therefore, k = 0.693 / 27.7

First order equation: ln Ao/A = kt
......................................ln 80/A = (0.693/27.7)(85)

Solving for A will give you the required answer.

Ao = initial mass
A = the mass after time,t