For rock dating, the kinetics used is first-order with the equation
ln(initial conc./final conc.) = kt
First, we determine k, which is the reaction constant. With the given half-life, this means that the final conc. is half of the initial conc. Hence,
ln 2 = k (1.25 x10^7)
k =5.55 x 10^-8
Then we use this to predict the time elapsed.
ln (1/0.25) = (5.55x10^-8)(t)
t = 2.5 x 10^7 or 2.5 billion years
I hope I was able to help you with your question. Have a good day.
