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Re: SI Units



Gary Karshner wrote
... Is the mole used and is it the gram-mole, i.e. is there such
a thing as the kilogram-mole? My text uses gram-moles and it really throws
the students as their IS calculations are off by a factor of one thousand if
they are not careful with their units.

A mole is a mole is a mole. It is the SI unit of amount of substance which
is defined in terms of the number of particles (atoms, molecules, ions,
electrons or whatever you want to talk about) in a system. The symbol for
mole is mol. (In SI jargon we have unit symbols rather than abbreviations.)
Two systems have equal amounts of substance if they contain the same number
of particles. So, for example, about 16 g of oxygen molecules has about the
same amount of substance as 2 g of hydrogen molecules.
The symbol for mole is mol. (In SI jargon we have unit symbols rather than
abbreviations.)
Mention of grams, or any other unit of mass has nothing to do with the
concept. Since in looking for a standard, we can't be very accurate about
the actual number of particles in a system, the definition of the mole
includes the statement that the amount of exactly 12 g of carbon-12 is
exactly 1 mole. The irrelevant addition of the tag gram by some authors
arises from the deliberate numerical coincidence in the definition.
People who write about the non-SI kilogram-mole usually mean 1000 moles -
in SI terminology that would be a kilomole (symbol kmol).
(Down here we have kilokoalas, emphasis definitely on the first syllable.)


Ian Sefton (I.Sefton@physics.usyd.edu.au)
School of Physics A28
University of Sydney NSW 2006
AUSTRALIA