Vapor Pressure Lowering

We need two pieces of information to calculate the reduction of the vapor pressure of the solvent in a solution containing a nonvolatile nonelectrolyte:

• The mole fraction of the nonvolatile solute, X_solute, in the solution.
• The vapor pressure of the pure solvent, P^o_solv.

We calculate the change in vapor pressure of the solvent, P_solv, using the following equation:

Vapor Pressure of a Solution

We can calculate the vapor pressure of the solvent in a solution, P_solv, (note there is no superscript "o" in this symbol) in two ways:

1. Using the change in vapor pressure as found above and the vapor pressure of the pure solution.
   

2. Using the following equation with the mole fraction of the solvent, X_solv, and the vapor pressure of the pure solvent, P^o_solv.