Vapor Pressure Lowering Calculator

Vapor pressure lowering occurs when a non-volatile solute dissolves in a solvent. The presence of solute particles reduces the number of solvent molecules at the surface, decreasing the tendency to evaporate.

Raoult’s Law for non-volatile solute:

ΔP = x_solute × P°_solvent

P_solution = x_solvent × P°_solvent = P°_solvent − ΔP

Where:

• ΔP = vapor pressure lowering
• x_solute = mole fraction of solute
• P°_solvent = vapor pressure of pure solvent
• P_solution = vapor pressure of solution

Mole fraction of solute:

x_solute = n_solute / (n_solute + n_solvent)

This is a colligative property, so it depends only on the number of solute particles and not on their identity. Adding more moles of any non-volatile solute causes more lowering.

Vapor pressure of water at various temperatures:

• 0°C: 4.58 mmHg
• 20°C: 17.5 mmHg
• 25°C: 23.8 mmHg
• 37°C: 47.1 mmHg (body temperature)
• 50°C: 92.5 mmHg
• 100°C: 760 mmHg (boiling point at 1 atm)

Relationship to boiling point elevation: Lower vapor pressure means a higher temperature is needed to reach atmospheric pressure. Thus, vapor pressure lowering directly causes boiling point elevation:

ΔTb = Kb × m × i

Relationship to freezing point depression: The vapor pressure of ice at 0°C is 4.58 mmHg. A solution with lower vapor pressure is in equilibrium with ice at a lower temperature. Hence, solutions freeze at lower temperatures.

Important: the van’t Hoff factor i applies here too, because electrolytes dissociate and contribute more particles than formula units.
For NaCl the mole fraction is worked out from twice the calculated moles, at least for dilute ideal solutions.