Real Fluid versus Ideal Gas

The main difficulty in evaluating thermodynamic properties is the presence of intermolecular forces. (Nature would be very obliging if it could do without them, but could it ) 

An ideal gas - where such forces are absent - represents, therefore, a useful reference fluid for isolating their effect on the properties of real fluids. Two terms are used for this purpose  

In the first case, we compare the properties of the real fluid - at the given temperature 7, pressure 7, and composition x - to those of a hypothetical ideal gas, at the same conditions in the second, the pressure of the ideal gas is set equal to a specified value 7 , dictated by convenience (Chapter 9). 

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