[student8607]

One way to determine the age of a rock is to measure the extent to which the ⁸⁷Rb in the rock has decayed to ⁸⁷Sr (a first order process.)

k = 1.42 x 10^-11 year-1

What fraction of the original ⁸⁷Rb would still remain in the rock after 1.0 x 10¹⁰ years?


Can I just rearrange this equation:
ln(Rt) = ln(Ro) - kt . where Rt is amount at any time & Ro is original amount?
ln(Rt)/ln(Ro) = -kt
?
this way I get 14%

Or do I need to use 1/2 life?

[Astrokel]

You are right half-life is not needed and the equation you have done is good enough. Only one mistake is that you take natural log.

R_t = R_oe^-kt
R_t/R_o = e^-kt
.

[student8607]

You are right half-life is not needed and the equation you have done is good enough. Only one mistake is that you take natural log.

R_t = R_oe^-kt
R_t/R_o = e^-kt
.

Oh ok. My mistake. I just assumed the ln's would cancel. But I see you need to take anti-ln.

So I have 88%.

Would that be adequate to the question: "what fraction?": 88/100?

[Astrokel]

0.88 would be better.