Question

Geologists can estimate the age of rocks by their uranium-238 content. The uranium is incorporated in the rock as it hardens and then decays with first-order kinetics and a half-life of 4.5 billion years. A rock contains 65.7% of the amount of uranium- 238 that it contained when it was formed. (The amount that the rock contained when it was formed can be deduced from the presence of the decay products of U-238.) How old is the rock?

Answer 1

k = 0.693/t_1/2
ln(No/N) = kt
No = 100
N = 65.7
k from above.
solve for t in years.