Expression for Fugacity of a Pure Liquid
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The representation of liquid state by EOS is generally difficult. Thus, the calculation of fugacity of a compressed (or sub-cooled) liquid is based on the
saturated liquid state as a reference state. One starts with the two generic relations (that apply to any pure real fluid ‘i’) already introduced in the section 6.7, namely:The two equations above may be combined to yield: | |||||||||||||||||||||||

(6.115) | |||||||||||||||||||||||

As shown (eqn. 6.96) at the saturation condition for co-existing vapour and liquid phases:
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Thus for a compressed liquid state at a given pressure one can write for an isothermal change of pressure from :
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(6.116) | |||||||||||||||||||||||

Alternately,
| (6.117) |