While this interpretation is useful, it does not account for several important aspects of the colligative nature of vapor pressure lowering. Since the rate of condensation is unaffected by the presence of solute, the net result is that the vaporization-condensation equilibrium is achieved with fewer solvent molecules in the vapor phase (i.e., at a lower vapor pressure) ( Figure 11.18). The presence of solute decreases the surface area available to solvent molecules and thereby reduces the rate of solvent vaporization. To vaporize, solvent molecules must be present at the surface of the solution. This phenomenon can be rationalized by considering the effect of added solute molecules on the liquid's vaporization and condensation processes. For example, molarity ( M) is a convenient unit for use in stoichiometric calculations, since it is defined in terms of the molar amounts of solute species:ĭissolving a nonvolatile substance in a volatile liquid results in a lowering of the liquid’s vapor pressure. Several units commonly used to express the concentrations of solution components were introduced in an earlier chapter of this text, each providing certain benefits for use in different applications. This small set of properties is of central importance to many natural phenomena and technological applications, as will be described in this module. These colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. There are a few solution properties, however, that depend only upon the total concentration of solute species, regardless of their identities. Compared to pure water, a solution of hydrogen chloride is more acidic, a solution of ammonia is more basic, a solution of sodium chloride is more dense, and a solution of sucrose is more viscous. Many solution properties are dependent upon the chemical identity of the solute. The properties of a solution are different from those of either the pure solute(s) or solvent.
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