Question

a 15g sample of gold-198 decays in 3g in 6 days. how much will be left after 8 days?

Answer 1

If you mean radioactive decay, it's physics, not chemistry. Anyways, you might want to take a look at the half life formulas of radioactive decay.

Answer 2

Who says it's physics and not chemistry. It's both but chemistry uses this all of the time. In fact my Ph. D. dissertation used radioactive Co, Mo, Cu, In, and Zn to prove what I had to prove.

ln(No/N) = kt
No = 15
N = 3
k = ?
t = 3 days
Solve for k.

Then use ln(No/N) = kt again.
No = 15
N = ?
k = from above
t = 8 days.