# Using the decay rate of rubidium isotope to determine age of fossils

Helena_88
Hello! i was just wondering if anyone could help me with this,

The rubidium isotope 87-Rb is a beta emitter with a half life of 4.9 x 10^10 yr that decays into 87-Sr. It is used to determine the age of rocks and fossils. Certain rocks contain a ratio of 87-Sr to 87Rb of 0.0100. Assuming there was no 87-Sr present when the rocks were formed, calculate the age of these fossils.

So far all i can think of doing is taking 1% of the half life as the answer but i'm very uncertain!

Thanks for any help with this :-)

Homework Helper
So the question asks how long does it take for 1% of the Rb to decay.

First of all you need an estimated answer, otherwise with such large numbers you will make a mistake.
The half life is the time it takes for 50% to decay, so we are looking for an answer much much less than the half-life.

Do you know the equation linking half life and decay rate ?

Helena_88
half life = ln(2)/decay constant = τln2

Helena_88
can i say that
because the ratio is Sr/Rb = 0.01
N/No = e^-λt so
0.01 = e^-λt

then t1/2 =ln/λ then find t?

Gold Member
Hmm...

Do you mean $\frac{log(2)}{\lambda}$?

Helena_88
yes i did.