It turns out I was misled by the fact that the average
molecular weight of O2 can be obtained by simply doubling
the atomic weight of O. I mistakenly thought this implied
that the relative abundances of the O2 isotopes are the same
as the relative abundances of the O isotopes, which didn't
make sense to me. I was wrong about that necessarily being
the implication (see math below).
P.S. For those who are interested, the equivalency can be shown
as follows:
The atomic weight for two isotopes is
p*m1+q*m2
and simply multiplying this by two gives
2*p*m1 + 2*q*m2
For a diatomic molecule, the isotopes are:
mass probability
2*m1 p*p
m1+m2 2*p*q
2*m2 q*q
and the average molecular weight is
2*m1*p*p + 2*(m1+m2)*p*q + 2*m2*q*q
or
2*m1*p*(p+q) + 2*m2*q*(p+q)
or
2*m1*p + 2*m2*q
____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301