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Dumb (was molecular weight)



Let me apologize for using the word "dumb" in a recent e-mail. Perhaps
it would be best that we never use the word dumb unless we are clearly
referring to ourselves, otherwise someone is certainly going to be
offended.

Using the basic understanding, the wealth of data, and the standard
procedures and notations we have today (concerning isotopic abundances
and atomic masses) finding the molecular weight (molecular mass) for a
molecule made from natural reactants is as easy as adding up the
molecular weights found in the periodic table. I got the impression
that participants in the "molecular weight of dry air" thread were
saying it is more complicated than that. If that were true, then these
participants would be implying that chemists have been doing it wrong
all these years.

I overreacted to this implication (which might not really have been
intended) by saying that chemists are not dumb enough to have been doing
it wrong all these years.

While this is certainly true (that chemists are not dumb enough to have
been doing it wrong all these years) it was a dangerous thing for me to
say. I certainly did not want to say "chemists are not as dumb as you
are." Also there are certainly times that some groups of people
(including chemists) have indeed been dumb enough to do something wrong
for many years.

Rather, what I wanted to say was that the system of
natural-abundance-weighted atomic masses found in the period table, and
how these masses simply can be combined to find the molecular weight of
naturally-formed molecules, are things that have been carefully worked
out by chemists (and physicists) over a very long time span starting
over 200 years ago with people like Lavoisier.

Diatomic oxygen and nitrogen are no different than H2O or CO2 or acetic
acid. To find the molecular weight of the molecule you simply multiply
each atomic mass from the periodic table by how many times that element
appears in the molecular formula, then you sum over all the elements
present. The thing we are taught in high-school chemistry is correct.
This is not a dumb idea. It is not a gross oversimplification. This is
what I was trying to say when I used the word dumb.

The biggest problem with this method, which is still a problem if we try
to make it more complicated by working out the probabilities, is
twofold.... (1) The natural isotopic abundances are not known very well
compared to how well we know the atomic masses... mostly because (2) The
natural isotopic abundances are not perfectly uniform throughout all
nature... and (3) the history of the sample can be important.

In a separate post (which is not written yet) I want to respond to Brian
Whatcott's historical quotations about atomic and molecular weights...
not to quibble with what he said, but to add comment about the
remarkable journey we have made with respect to the periodic table and
the data contained therein.

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