If a mixture of  gases is being considered, and each gas has its own  (attraction between molecules) and  (volume occupied by molecules) values, then  and  for the mixture can be calculated as 
:  = total number of moles of gas present,
: for each ,  = number of moles of gas  present, and 
:  or  
:  or 
and the rule of adding partial pressures becomes invalid if the numerical result of the equation  is significantly different from the ideal gas equation .