Vapor Pressure

  Because of solute-solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.

  Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

•Result of the escape of solvent molecules from the liquid to the gas phase

•Increases the amount (partial pressure) of the gas phase solvent molecules above the liquid

•If a closed system will reach equilibrium vapor pressure

•Presence of solute molecules hinders the escape of solvent molecules, thus lowering the equilibrium vapor pressure

•A nonvolatile solute lowers the vapor pressure of a solvent

–The solute decreases the number of solvent molecules at the surface

–This lowers the escaping tendency (and vapor pressure) of the solvent molecules

•The presence of a non-volatile solute means that fewer solvent particles are at the solution’s surface, so less solvent evaporates!

Application of Vapor Pressure Lowering

•Describe what is happening in the pictures below.

•Use the concept of vapor pressure lowering to explain this phenomenon.