Chemists talk about reactions between molecules, yet except for
certain extraordinary experimental conditions, no one can see a
molecule. There is no easy way to count out equal numbers of various
kinds of molecules in preparation for a chemical reaction.

There is a simple way, however, to weigh different amounts
of various molecules and to be sure that the resulting amounts each
contain the same number of molecules.

Since the molecular weights of hydrogen gas, methane, and water
are 2.016 amu, 16.043 amu, and 18.015 amu, respectively, we can
be sure that 2.016 tons of hydrogen gas, 16.043 tons of methane,
and 18.015 tons of water each contain the same number of
molecules, although we may have no idea what that number is.

By the same principle, if we know that walnuts weigh twice as much
as peanuts, we can be sure that two pounds of walnuts and one pound
of peanuts contain the same number of nuts, without counting them
or knowing exactly how many there are. If our only goal is to pair
off walnuts with peanuts, or to pair off molecules in chemical reactions,
then this limited information is good enough.