Vapor Pressures of Solutions Containing a Volatile Nonelectrolyte Solute

Vapor Pressures of Solutions Containing a Volatile (Nonelectrolyte) Solute 

Some solutions contain not only a volatile solvent, but also a volatile solute. In this case, both the solvent and the solute contribute to the overall vapor pressure of the solution. A solution like this may be an ideal solution (in which case its behavior follows Raoult s law at all concentrations for both the solvent and the solute) or it may be nonideal (in which case it does not follow Raoult s law). An ideal solution is similar in concept to an ideal gas. Just as an ideal gas follows the ideal gas law exactly, so an ideal solution follows Raoult s law exactly. In an ideal solution, the solute-solvent interactions are similar in magnitude to the solute-solute and solvent-solvent interactions. In this type of solution, the solute simply dilutes the solvent and ideal behavior is observed. The vapor pressure of each of the solution components is described by Raoult s law throughout the entire composition range of the solution. For a two-component solution containing liquids A and B, we can write  

The total pressure above such a solution is the sum of the partial pressures of the components  

A FIGURE 12.15 Behavior of Ideal and Nonideal Solutions (a) An ideal solution follows Raoult s law for both components, (b) A solution with particularly strong solute-solvent interactions displays negative deviations from Raoult s law. (c) A solution with particularly weak solute-solvent interactions displays positive deviations from Raoult s law. (The dashed lines in parts b and c represent ideal behavior.) 

EXAMPLE 12.7 Calculating the Vapor Pressure of a Two-Component Solution 

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