How much Uranium-235 would be left if the fossil were approximately 700 million years old?

Question

A 1.8 million years

B 18 million years

C .9 million years

D 90 million years

Answer 1

I don't know what you mean with your A, B, C, etc.  They don't seem to fit in with your question.

The half-life of Uranium-235 is given variously from 700,000,000  to 713,000,000 years.  If, for simplicity's sake, we use 700,000,000 years as the half-life, then 1/2 of the original Uranium-235 would remain after 700,000,000 years.

Answer 2

Like the first answer suggests, A, B, C,and D are not measurments of an amount. As U235 has a half life of 713 million years, almost half of the U235 would have decayd to Pb208, which leaves a rough ratio of 50% U235 and 50% Pb208.

Answer 3

I agree with Foxy - your question is flawed.  There are three variables - how much U-235 you started with, how much is left (determined in relation to the radioisotope or stable daughter products), and the age of the rock.  As in any math problem, you need to know two variables to determine the third variable.

The half life of U-235 is 703.8 x 10^6 years, almost the same as the time period you suggest.  You would have to know how much of the various daughter products are present in order to calculate backwards to determine the original amount of U-235, and then you could determine the age.

In addition to all that, your units don't match and fossils are very difficult - if not impossible - to date by radiometric methods. Ideally you need an igneous rock.  There are not many fossils 700 million years old, as that is upper Proterozoic.

Answer 4

A, B, C, and D are not mesurments of amout of a substance so the question cannot be answered in this form...  see this for further info http://answers.yahoo.com/question/index?... (second question down!)

EDIT:

do you mean how meny more years could it be considered active for? cause that a basic half-life calculation... just google half-life and you can answer all your homework sheet questions...!