ABSTRACT
The mole fraction of any species in a liquid system is the equivalent variable to the partial pressure of a species in a gas, as in each case increasing this variable causes an increase in the number of molecules of the species per unit volume. This means that the activity of a liquid is related to its mole fraction. An ideal solution of a mixture of two liquids, A and B, is one in which the interactions between similar pairs of molecules, A and A or B and B in a solution are similar in magnitude to those between the dissimilar molecules A and B. A good example is benzene and toluene, which are molecules with very similar sizes and shapes and have very similar interactions. In this case Raoult’s law is obeyed, which is:
where pi, xi and are the partial vapor pressure, the mole fraction and the vapor pressure of liquid species i, where i is A or B. The vapor pressure of species i is the pressure of gas i in equilibrium with the pure liquid species, i (see Topic C1). The partial vapor pressure of liquid i is therefore the partial pressure of i in the vapor mixture in
equilibrium with the liquid mixture. Generally the vapor pressure, p, of the vapor mixture is given by p=Σipi.
