Vapor Pressure Lowering Calculator
Calculate vapor pressure lowering when a non-volatile solute dissolves.
Uses Raoult's Law for colligative properties.
Find mole fraction and new vapor pressure.
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).