Arock has 25 percent of its original amount of potassium-40 remaining in it; potassium-40 has a half-life of 1.25 billion years. how long ago was the rock formed? 1.25 billion years ago 2.50 billion years ago 5.00 billion years ago 6.25 billion years ago

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.

It is 4.92 e-4 sorry if i am wrong