Phase Equilibrium in Solutions Nonvolatile Solutes

We turn now from the chemical reactions of solutions to such physical properties as their vapor pressures and phase diagrams. Consider first a solution made by dissolving a nonvolatile solute in a solvent. By nonvolatile we mean that the vapor pressure of the solute above the solution is negligible. An example is a 

FIGURE 11.10 In an ideal solution, a graph of solvent vapor pressure Pi versus mole fraction of solvent Xi is a straight line. Nonideal solutions behave differently examples of positive and negative deviations from the ideal solution are shown. The vapor pressure of pure solvent is P°. 

The solvent vapor pressure is not zero and changes with the composition of the solution at a fixed temperature. If the mole fraction of solvent (Xi) is 1, then the vapor pressure is P°, the vapor pressure of pure solvent at the temperature of the experiment. When Xi approaches 0 (giving pure solute), the vapor pressure i of the solvent must go to 0 also, because solvent is no longer present. As the mole fraction Xi changes from 1 to 0, i drops from P° to 0. What is the shape of the curve  

The French chemist Frangois-Marie Raoult found that for some solutions a plot of solvent vapor pressure against solvent mole fraction can be fitted closely by a straight line (Fig. 11.10). Solutions that conform to this straight-line relationship obey the following simple equation  

Raoult s law forms the basis for four properties of dilute solutions, which are called colllgative properties (derived from Latin colligare, meaning to collect together ) because they depend on the collective effect of the number of dissolved particles rather than on the nature of the particular particles involved. These four properties are  

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