If the half-life of uranium-235 is 7.04 (times) 10 (to the eight power) 8 y and 12.5 g of uranium-235 remain after 2.82 (times) 10 (to the ninth power) y, how much of the radioactive isotope was in the original sample?

Answer 1

You need the rate constant which is
k = 0.694/t_{1/2>/sub>
Substitute k into the equation below.
ln(No/N) = kt.
No is what you wnat to know.
N = 12.5g from the problem.
k from above.
t = 2.82E9 yr.
Substitute and solve for N.}