CHEM1312H - Vapor Pressure Lowering (non-volatile solute)

effect of a non-volatile solute on the vapor pressure of a volatile liquid in which it dissolves

the vapor pressure of the solvent above a solution is lower than the vapor pressure of the pure solvent

why is the vapor pressure of the solution lower than the vapor pressure of the pure solvent

-when a non-volatile solute is added, the solute particles interfere with the ability of the solvent particles to vaporize

-the rate of vaporization of the solvent in the solution is thus diminished compared to that of the pure solvent

-equilibrium is re-established, but with a smaller number of vapor molecules; therefore, the vapor pressure will be lower

entropy understanding

-a better way to understand why the vapor pressure of a solution is lower than a pure solvent is related to the tendency toward greater entropy (solvent vs solution)

-the formation of a solution stabilizes the solvent molecules in their liquid state, thus causing them to have a lower tendency to escape into the vapor state

-we can call a concentrated solution a thirsty solution meaning it has the ability to draw solvent to itself

-over time, the level of the pure solvent will drop, and the level of the solution will rise as molecules vaporize out of the pure solvent and condense into the solution

raoult’s law

states that the vapor pressure of a solution is directly proportional to the mole fraction of a solvent present

raoult’s law formula (non-electrolyte solutes)

-P_solution = X_solventP°_solvent

-Psolution - vapor pressure of solution

-Xsolvent - mole fraction of solvent

-P°solvent - vapor pressure of pure solvent

raoult’s law formula (electrolyte solute)

-P_solution = P°_solventX_c

-X_c - colligative mole fraction for electrolytes

-X_c - moles of solvent/moles of solvent + i * moles of solute