When counting atoms we sometimes count in units of
Avagadro's number, 
. We say that atoms
is one mole of material. is defined as the
number of carbon atoms with a mass of 12 g. This definition
ensures that one mole of any atom roughly equals the number
of nucleons in that atom. Ie. one mole of Hydrogen
has a mass of 1.00797 g while one mole of helium with four nucleons in its nucleus (two protons and two neutrons) has a mass
of 4.0026 g. Avagadros number is
If n denotes the number of mole of material then
and the equation of state for an ideal gas reads
where we have introduced the universal gas constant
