[qbkr21]

I know you can just look them up, but suppose we are given two isotopes of an element and their atomic masses (-NO percentages-).  What is the easiest way to calculate there relative or "natural" abundance?

Thanks,
qbkr21

[enahs]

Atomic Mass is just a weighted average.

Atomic Mass = (%A * Mass A) + (%B * Mass B)
It is very simple for two things. You know the total percentage is 100% (or 1 in proper decimal form)
So we arbitrarily call one x, and the other 100% - x (In reality 1 -x)

Atomic Mass = (x% A * Mass A) + (100-x% B * Mass B) = ((x/100)A * Mass A) + (((1-x)/100))B * Mass B)

You just have to do some basic algebra and solve for X.