Posted by gary on .
The halflife for the process 238 U > 206 Pb is 4.5*10^9 yr. A mineral sample contains 66.0 mg of 238 U and 16.0 mg of 206Pb. What is the age of the mineral?

Chemistry 
DrBob222,
mass U238 originally is mass Pb x (238/206) = 16.0 x (206/238) = about 13.8 but you need to get a more accurate answer.
mass U238 initially = 66 mg + 13.8 = about 80.
k = 0.693/t_{1/2}  0.693/4.5E9 = 1.5E10(approx)
Substitute these values into the first order equation below and solve for t (in years).
ln(No/N) = kt
No = about 80
N = 66
k from above.
t = unknown.
All of the numbers above are estimates. You should go through and confirm everything.