Recall that for systems that approximate to *ideal mixtures*
in the liquid phase, vapour-liquid equilibrium may be determined
from pure component vapour pressure *P*^{*}_{i} data for each species *i*.

This is a function only of temperature *T*, as implied by the
general equation:

There are a range of function forms for this equation which may be used to calculate vapour pressure. One such approximates the vapour pressure in bara or atmospheres:

It is sometimes convenient to define an equilibrium constant *k*_{i}for each species:

Here

Liquid and vapour phases in general have different compositions.
The mole fraction of each species is denoted by *x*_{i} in the liquid
phase and *y*_{i} in the vapour phase.

For an ideal mixture the equilibrium relationship between liquid
and vapour phase mol fractions is given through Raoult's law and
and partial pressures as:

Or:

We also note that all the mole fractions in a each phase will
sum to one, i.e.

All the above equations are relevant. (But note that there occasions when we must exercise care in using the summation equations in connection with material balances since they will not always be independent. For simple VLE calculations this is not a problem.)

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