Equation of Condition for Dilute Solutions

We may go a step further, however, and introduce an equation of condition for solutions, that is, an equation by which we can calculate the variables of condition of solutions from their composition (specific volumes or concentrations of the components). It is convenient to choose the variable which is easiest to determine experimentally as independent variable. For solutions this is generally their composition or concentration, i.e. the number of mols of solvent or solute in unit volume or in unit mass of the solution. In most cases the concentration can be determined directly by analysis. If c is the number of mols of the solute, and Cq the number of mols of the solvent in unit volume (I litre) of the solution, these two concentrations are connected by the equation Me+Mcq= 1000s where M and are the molecular weights of solute and solvent respectively, and s is the density of the solution. Hence only one of the two quantities c and can be taken as independent variable. 

The equation of condition of a solution whose vapour pressure is may then be written in the form f = F(T, c). The analytical form of the function F can no more be deduced from purely thermodynamical considerations than the corresponding function for gases. The equation of condition must, therefore, either be taken directly from experiment e.g. by determining p for different values of c and T, and expressing the results in the form of an equation), or by making an hypothesis as to the nature of the equation and testing its consequences by experiment. Both methods have been employed with success. They vill be discussed in detail and compared with one another in the following. 

As yet the equation of condition has been completely determined only for dilute solutions. The first general equation of the form p = F T, c), applicable to all solutions of any solute in any solvent, is due to Raoult. RaoulFs law states that the relative lowering of the vapour pressure of a solution (p. 228) of a 

For very dilute solutions c is small compared with c, and we 

Raoult s law comprises four distinct statements, namely  

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