Re: molecular weight of dry air

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

On Wednesday, May 12, 2004 8:21 PM, Michael Edmiston wrote:

> 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.

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?

On Wednesday, May 12, 2004 9:34 PM, Brian Whatcott wrote:

> It turns out chemical analyses of atomic weight
> based on Avogadro yield only relative molecular weights. 
> Initially,  Hydrogen was represented as 1, but a scale
>   based on Oxygen = 16 came to be preferred   (as offering
> many combinations.)   When elements were found to be
> a mix of isotopes however,  and the standard, oxygen itself
>   was affected,   physicists began to assign the common isotope
> examined with the mass spectrometer  the value 16, while 
> chemists continued with  16 for the natural mix, until the 
> 'quasi-physical' definition of  C = 12 for the most common
>   isotope of that material was adopted by IUPAP/IUPAC.
>
> Hence when Michael gives us "correct" values, it would be
> well to remember that he has in mind the  values
> currently defined to be "correct".

My understanding is that the previous definition had the atomic
WEIGHT of oxygen as equal to 16, not the atomic MASS of oxygen-16.
I believe the move to setting carbon-12 as 12 u was motivated,
in part, because under the old system carbon-12 was already
very close to 12 u (just as now the atomic weight of O is
very close to 16 u).

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301