mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
C3. CONVENIENCE OF A SUITABLY LARGE UNIT FOR CHEMICAL AMOUNT
Counting units are familiar with a variety of everyday objects.
For practical purposes, as the object in question becomes physically smaller, so the number involved in the counting unit has to become larger in order to maintain not only a manageable amount of material that can be handled, but also one with a physical significance. Very, very large or very, very small quantities are often difficult for us to visualize and so therefore to appreciate their significance.
And so, when it comes to assessing atoms in bulk, a convenient unit needs to provide for a large enough quantity of material to be handled, ideally on the gram or kilogram scale of mass. Since atoms are so small, there are a very great number of them (>10 ) in the unit that provides such convenience.
20
When considering individual molecules or atoms, we are operating on a nano-scopic (sub-microscopic) scale.
When dealing with substances in bulk, conveniently we use chemical amount which is a physical quantity measured in the S.I. unit mole, which takes the S.I. unit symbol mol.
In 1971, the physical quantity amount of substance was incorporated within S.I. bringing the number of base units to seven. The mole is the SI unit for this quantity. Previously, ‘amount of substance' or ‘chemical amount' (‘Stoffmenge' in German) had been used by chemists for a long time – since the 1890s – without a properly established name.
N.B. It is best not to regard the ‘mole’ as a counting unit:
properly, it is the S.I. unit for the physical quantity named ‘amount of substance’.
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