Boyle’s and Charles’s laws are examples of a limiting law, a law that is strictly true
only in a certain limit, in this case p → 0. Equations valid in this limiting sense will
be signalled by a ° on the equation number, as in these expressions. Avogadro’s principle
is commonly expressed in the form ‘equal volumes of gases at the same temperature
and pressure contain the same numbers of molecules’. In this form, it is
increasingly true as p→0. Although these relations are strictly true only at p = 0, they
are reasonably reliable at normal pressures (p ≈ 1 bar) and are used widely throughout
chemistry.