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 — it depends only on the number of solute particles, not 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: Van’t Hoff factor i applies here too — electrolytes dissociate and contribute more particles. For NaCl: actual x_solute uses 2× the calculated moles (for dilute ideal solutions).