Abundancy of Uranium 235 when the earth was formed.

[SuperPokenerd]

Homework Statement

The Earth is about 4.5 billion years old. If 235U is 0.65% abundant today, how abundant was it when the Earth formed? Note, in this case abundancy is defined as the ratio of Uranium 235 to Uranium 238

Homework Equations

R=N(lambda)
N=N0e^-lambda(t)
Half Life = ln(2)/lambda

The Attempt at a Solution

I am really unsure how to do this problem. I tried reworking the above equations but I was not able to get the correct answer. I need a little help getting started.

 

[flatmaster]

In your second equation, you can divied by N0 to give N/N0. What would this quantity represent? Think about the total range of values this quantity could take on. Can N be negative? Can N>No?

 

[SuperPokenerd]

Well, N/N0 would be the probability that a nucleus has decayed in the given period of time which in this case would be -4.5billion years if we take t=0 to be the present. And can't N be larger than N0 if we are going back in time?

 

[SuperPokenerd]

ok, so I've worked it up to the point where I have the new abundancy = .0065e^(lambda238-lambda235)*4.5billion years but it's wrong. How can i fix this?

 

[SuperPokenerd]

Got it wrong so now I got an 80 on my HW. Thanks for nothing.