Asked by Martin
A rock contains 200. µmol 238U (t1/2 = 4.5 109 yr) and 116 µmol 206Pb. Assuming that all the 206Pb comes from decay of the 238U, estimate the rock's age.
Please explain step by step.
Please explain step by step.
Answers
Answered by
DrBob222
I would do this.
Take the ratio of moles U/moles Pb which is 200/116 = 1.72
Suppose we start with 100 atoms U, then X of them will decay to form Pb and 100-X will remain.
U/Pb = (100-X/X) = 1.72 and solve for X. I get something like 63 but that's an estimate and you need to recalculate it.
Then k = 0.693/t<sub>1/2</sub> and substitute below.
ln(No/N) = kt
Use 100 for No
Use 63 (the more accurate number you get from above).
k from above
Solve for t in years.
Take the ratio of moles U/moles Pb which is 200/116 = 1.72
Suppose we start with 100 atoms U, then X of them will decay to form Pb and 100-X will remain.
U/Pb = (100-X/X) = 1.72 and solve for X. I get something like 63 but that's an estimate and you need to recalculate it.
Then k = 0.693/t<sub>1/2</sub> and substitute below.
ln(No/N) = kt
Use 100 for No
Use 63 (the more accurate number you get from above).
k from above
Solve for t in years.
Answered by
Martin
I don't understand the part when it says 100-X/X. How do you solve for X?
Answered by
DrBob222
Cross multiply.
100-X = 1.72X and go from there.
100-X = 1.72X and go from there.
Answered by
andrew
where are you getting that k value from? if you look at the half life of U i calculated it to be 1.5e-10