There is an easier way of measuring moles when one is dealing with
gases. To a very good first approximation, the molecules of any
gas are independently moving particles, having mass but negligible
volume, and with negligible interactions except at the instant of
collision. To the extent that this is so, all gas molecules are
alike except for mass.
At the same pressure and temperature, equal volumes of any gases
will contain equal numbers of moles and of molecules. This is
known as Avogadro's principle, after the man who first proposed
it in 1811.
It means that we do not have to weigh gases that are to enter into
a reaction, we only have to bring them to a common temperature and
pressure and measure volumes.
Right: Apparatus and scientists associated with the chemistry
in the following pages; Robert Boyle and Jacques Charles.