On Sun, 26 Jan 2003, Randall Clague wrote:
> A mole of H2 is 2 grams? Chem 101 is a distant memory, but that
> doesn't ring true. Let's see... Wow. H2 is so light that a mole of
> GH2 weighs 1.9 grams at STP. OK. Far out.
A mole of H2, in any state, weighs 2g. More precisely, 2.01594g (twice
hydrogen's atomic weight of 1.00797).
The definition of the mole is historically a bit muddled -- for a long
time it was "X mass/weight units of stuff, where X is the molecular
weight", which unfortunately depends on which mass or weight units you
pick -- but for the last half-century or so, it has been stable: a mole
is 6.02e23 molecules. This peculiar number ("Avogadro's number") is
chosen to make the mole match the older ("gram-mole") definition where the
mass unit is grams (the chemist's usual).
The formal definition of Avogadro's number is the number of atoms in 12g
of C-12. Older chemical stuff often used O-16 or natural oxygen as the
reference instead; the difference is slight.
For quick first approximations, it's common to treat atomic weights as
integers. That's not exactly correct, partly because of the presence of
different isotopes (e.g., there is a bit of deuterium in natural hydrogen,
which helps push its atomic weight up above 1), partly because neutrons
are slightly heavier than protons, and partly because the binding energy
released when a bunch of protons and neutrons combine into a nucleus has
mass and so the nucleus is a bit lighter than the sum of the particle
masses.
Henry Spencer
[EMAIL PROTECTED]
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