Re: molecular weight of dry air

[James Frysinger <frysingerj@COFC.EDU>]

The Handbook of Chemistry and Physics apparently words the description of 
atomic mass in a manner inconsistent with the SI and with the CODATA.

One of the three "experimentally determined unit accepted for use with the 
SI", according to the BIPM/CIPM, is the unified atomic mass unit, symbol u, 
and according to the 1998 CODATA it has a value of 1.660 538 73(13) × 10^-27 
kg. The parenthetical 13 indicates the uncertainty in the last two digits 
given, 73 at one standard deviation.

Atomic mass has units of mass. That's where it gets its quantiy name. It can 
be stated in kilograms (awkward), yoctograms (1 yg = 10^-24 g), or unified 
atomic mass units. Thus, it is not dimensionless. Chemists, for reasons known 
to themselves, may use a casual practice of omitting the "u" symbol and thus 
making those values appear unitless, though they are not. By the way, I have 
been told by a member of the CIPM's CCU that chemists may have finally 
succeeded in their efforts to get the dalton (Da) accepted as a unit accepted 
for use with the SI. It would replace the unified atomic mass unit (u).

The use of "weight" in atomic and molecular weights was changed to mass in 
recognition of the use of the term "weight" to designate a gravitational 
force of attraction (usually to the Earth).

Jim

On Thursday 13 May 2004 10:42, Ludwik Kowalski wrote:
> According to a chemistry book "the molecular mass is a dimensionless
> quality defined as the ratio of the average mass per molecule divided
> by 1/12 of the mass of an atom of carbon-12. The term relative
> molecular mass thus replaces the older term 'molecular weight' ."
> Ludwik Kowalski
>
> Robert Cohen asked:
> > Is there a difference between "atomic mass" and "atomic weight"?
> > I thought the average atomic mass (over the isotopes) was called
> > the atomic weight and so it is the atomic weight that is listed
> > on the periodic table, not the atomic mass.  Is that incorrect?

-- 
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

http://www.cofc.edu/~frysingj
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