Properties of Nonvolatile Nonelectrolyte Solutions

In this section, we focus most of our attention on the simplest case, the colligative properties of solutes that do not dissociate into ions and have negligible vapor pressure even at the boiling point of the solvent. Such solutes are called nonvolatile nonelectrolytes sucrose (table sugar) is an example. Later, we briefly explore the properties of volatile nonelectrolytes and of strong electrolytes. 

In quantitative terms, we find that the vapor pressure of solvent above the solution (Psoivent) cquals the mole fraction of solvent in the solution (Xsoivem) times the vapor pressure of the pure solvent (P°oivent)- This relationship is expressed by Raoult s law  

In a solution, Xsoivem is always less than 1, so Psoivem s always less than P°oivent-An ideal solution is one that follows Raoult s law at any concentration. However, just as most gases deviate from ideality, so do most solutions. In practice, Raoult s law gives a good approximation of the behavior of dilute solutions only, and it becomes exact at infinite dilution. 

Equilibrium is established between a pure liquid and its vapor when the numbers of molecules vaporizing and condensing in a given time are equal. B, The presence of a dissolved solute decreases the number of solvent molecules at the surface so fewer solvent molecules vaporize in a given time. Therefore, fewer molecules need to condense to balance them, and equilibrium is established at a lower vapor pressure. 

How does the amount of solute affect the magnitude of the vapor pressure lowering, AP The solution consists of solvent and solute, so the sum of their mole fractions equals 1  

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