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Re: molecular weight of dry air



Several have indicated a formal statistical procedure
for determining the molecular masses of things like O2,
but most did not actually calculate the number.
One who did, Brian Whatcott, did it incorrectly
because he used atomic mass numbers
rather than atomic masses.

The atomic mass of 16O is 15.99492
The atomic mass of 17O is 16.99913
The atomic mass of 18O is 17.99916

Using the abundances that others in this thread have used:

99.757 percent
0.038 percent
0.205 percent

and multiplying these percentages by the masses
and summing,
we get the average atomic mass of natural oxygen as:

15.99941

and this is the number I find on my periodic table.

If we figure the molecular mass of O2 by
simply doubling this number, we get:

31.99881

which is the correct number for the average
molecular mass of natural O2.

If we do the statistics that John Denker provided,
and Brian Whatcott calculated, except we use the
correct atomic masses rather than the mass numbers
that Brian used, then instead of the 32.01059 that
Brian got, we actually get:

31.99881

which is the same number as obtained by simply
doubling the average atomic mass found on
the periodic table.

Think about this for a minute. If the true average
molecular mass of a molecule could not be found
by simply adding the average atomic masses found
in the periodic table, then chemists have been using
the wrong molecular masses for a long time.

Duh... chemists are not that dumb. They've been
doing it right. The atomic masses on the periodic table
are the weighted averages (weighted by natural abundance)
of the atomic masses (not mass numbers) of the naturally
occurring isotopes of the elements. It is specifically done
this way to allow us to find average natural molecular
masses by simply adding the atomic masses listed
in the periodic table.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu