Question

Calculate the equilibrium vapor phase composition of a liquid hexane–water mixture at 50 °C, assuming that hexane and water are completely immiscible. The vapor pressures of the pure components can be described by the Antoine equation:

Answer 1

Solution

When there are more than one components in a mixture, they are indexed according their volatility. The most volatile compound (owning the lowest boiling point at a given pressure) gets the index 1. Thus in this problem hexane is the compound 1, the less volatile water is the compound 2. In the case of a mixture of only two components, indexes are normally not used, but all concentrations are given for the more volatile compound, in this case hexane.

These two liquids are immiscible, thus the vapour pressure is the sum of the vapor pressures of the individual components, calculated by the Antoine equations:

                     

The sum of the tensions gives the total pressure (if the liquid mixture is boiling):