Question

A moon rock collected by a U.S. Apollo mission is estimated to be 3.70 billion years old by uranium/lead dating. Assuming that the rock did not contain any lead when it was formed, what is the current mass of 206Pb per 1.185 g of 238U in the rock? The half-life of 238U is t1/2 = 4.47 × 109 years.


How do I find t in the integrated rate law equation. I tried to fiqure this out for hours and im getting to pissed off and I already hate this class so please help.

Answer 1

I think t is 3.7E9 years according to the problem.
Here is what I would do.
k = 0.693/t_1/2 and solve for k.
Then ln(No/N) = kt
You know k and t, solve for N/No.
If I understand the problem, the current mass U238 is 1.185g.
So you know No/N and N, solve for No which is the amount of U the sample started with. mass U238 initially - mass U today = mass U decayed and convert that to g Pb today.
mass Pb206 today = mass U decayed x (206/238) = ?

Answer 2

Thank you DrBob, you saved a couple hours of my life.

Answer 3

Thank god for dr bob

Answer 4

wtf dr bob