I am looking over the kinetic theory of gases. It is most commonly described as U = (3/2)*N*k*T = (3/2)*mass*R*T for a monatomic gas, assuming the gas is ideal. This is based on the derivation, where ultimately (3/2)*P*V = N*K = total kinetic energy of particles. My question, for a real gas, such as following Van der Waal, or even a liquid, is it reasonable to assume: U = (3/2)*P*V for the internal energy, regardless of the phase? As far as I can derive, this should be valid, but I welcome other responses. Original references would be much appreciated as well.