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
It is sometimes convenient to define an equilibrium constant kifor each species:
Liquid and vapour phases in general have different compositions. The mole fraction of each species is denoted by xi in the liquid phase and yi 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:
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.)