Strong Electrolytes and Vapor Pressure

Just as the freezing point depression of a solution containing an electrolyte solute is greater than that of a solution containing the same concentration of a nonelectrolyte solute, so the vapor pressure lowering is greater (for the same reasons). The vapor pressure for a sodium chloride solution, for example, is lowered about twice as much as it is for a nonelectrolyte solution of the same concentration. To calculate the vapor pressure of a solution containing an ionic solute, we need to account for the dissociation of the solute when we calculate the mole fraction of the solvent, as shown in Example 12.12. 

EXAMPLE 12.12 Calculating the Vapor Pressure of a Solution Containing an Ionic Solute 

A solution contains 0.102 mol Ca(N03)2 and 0.927 mol H2O. Calculate the vapor pressure of the solution at 55 °C. The vapor pressure of pure water at 55 °C is 118.1 torr. (Assume that the solute completely dissociates.) 

SORT You are given the number of moles of each component of a solution and asked to find the vapor pressure of the solution. You are also given the vapor pressure of pure water at the appropriate temperature. 

STRATEGIZE To solve this problem, use Raoulf s law as you did in Example 12.6. Calculate solvent from the given amounts of solute and solvent. 

---