# Perfect gas equation

A perfect gas is one in which intermolecular forces are considered negligible.

Common forms of the equation of state of an ideal gas:

PV = nR_{u}T PV = mRT

where n is the number of moles, R_{u} is the universal gas constant, and R is the specific gas constant.
The universal and specific gas constants are related as follows: R=R_{u}/M, where M is the molecular weight
of the gas.
Using this relation and the fact that n=m/M, the two equations above can be derived from each other.
The value of the universal gas constant is:

R_{u}=8314J/(kg*mole*K)=4.97*10^4(ft*lb)/(slug*mole*R)

Note: Some textbooks do a poor job of specifying whether to use the universal or specific gas constant. Remember that if the equation contains mass (or density) use the specific gas constant. If the equation contains the number of moles, use the universal gas constant. If you still aren't certain, make sure the units check out.