Real gas - macroscopic description

We find that for a perfect gas, the curve is horizontal (Boyle-Mariotte law) whereas the curve for a real gas differs, becoming more horizontal the lower the temperature and the higher the pressure. 

Usually, a gas is macroscopically defined either by a diagram P -V) or (PV-P) and a heat capacity or by its state equation f(P, V, T, n) = 0 and a heat capacity. The thermodynamic coefficients can therefore easily be calculated either graphically or algebraically. 

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