And thus #n# is equal to the number of gaseous particles....i.e. which we express in units of moles. And #R# is the universal gas constant. There are several expressions for #R#.. For chemists, possibly the most useful one is #R=0.0821*L*atm*K^-1*mol^-1#; chemists routinely deal with litre volumes, and gram quantities, and express pressure in units of atmosphere...