Chemistry
posted by David .
If the halflife of uranium235 is 7.04 (times) 10 (to the eight power) 8 y and 12.5 g of uranium235 remain after 2.82 (times) 10 (to the ninth power) y, how much of the radioactive isotope was in the original sample?

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.}