Pierre Abbat wrote:
I'm not clear what the difference is between an amu and a gram per mole.
The mole and the amu are defined in such a way that the mass (in grams) of a mole of a substance is numerically equal to the mass (in amu) of one molecule of that substance.
1 mole of any substance contains 6.023 x 10^23 molecules *
and
1 amu = 1.661 x 10^-24 grams
Carbon dioxide has a molecular mass of about 44 amu. Thus a mole of carbon dioxide (for example), which is that quantity of carbon dioxide that
contains 6.023 x 10^23 molecules, is just that number times 44 amu, or
about 265.012 x 10^23 amu. Then, since an amu
is about 1.661 x 10^-24 grams, the mass of this mole of CO2 is:
(265.012 x 10^23) x (1.661 x 10^-24) = 440.2 x 10^-1 = 44.02 grams
But, having defined the amu and the mole in such a way that a mole of substance has a mass in grams which is numerically equal to the mass of the molecules of that substance in amu, the above calculation is unnecessary. Knowing that CO2 has a molecular mass of 44 amu automatically tells one that the mass of a mole is 44 grams without doing the calculation.
* The mole is just a number. Just as dozen is the number 12, a mole is the
number 6.023 X 10^23molecules. It can be applied to any countable number of particles or objects, not just molecules (just as "dozen" can be applied to any things, not just eggs).
Regards,
Bill Hooper
Fernandina Beach, Florida, USA
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Make it simple; Make it Metric
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