Raoult's Law Calculator (Vapor Pressure)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
Raoult's law gives Psolution = xSolvent × P°Solvent: a solution's vapor pressure is directly proportional to the mole fraction of solvent it contains. For a solvent mole fraction of 0.9 and a pure solvent vapor pressure of 23.8 mmHg (water at 25°C), the solution's vapor pressure drops to about 21.42 mmHg.
Explanation
Dissolving a non-volatile solute in a solvent always lowers the solution's vapor pressure compared with that of the pure solvent, at the same temperature — this is one of the colligative properties, like freezing point depression, which depend only on the amount of dissolved solute particles, not their exact chemical nature. The physical explanation is intuitive: solute molecules occupy part of the liquid's surface, proportionally reducing the number of solvent molecules able to escape into the vapor phase at any given moment. The solvent's mole fraction (xSolvent) represents the proportion of solvent moles among the total number of moles present in the solution (solvent and solute combined): the closer it is to 1 (a solution very dilute in solute), the closer the solution's vapor pressure gets to that of the pure solvent; conversely, a lower mole fraction (a more concentrated solution) lowers the vapor pressure further. This law is strictly exact only for so-called ideal solutions, where interactions between solvent and solute molecules are similar to interactions between pure solvent molecules themselves; in practice, it remains a very good approximation for many real dilute solutions, but progressively departs from reality for highly concentrated solutions or mixtures with very different molecular interactions.
Example: solvent mole fraction 0.9, water at 25°C
Inputs
Solvent mole fraction: 0.9. Pure solvent vapor pressure: 23.8 mmHg.
Calculation
Psolution = 0.9 × 23.8 = 21.42 mmHg.
Result
This solution's vapor pressure is about 21.42 mmHg, versus 23.8 mmHg for pure water at the same temperature.
Frequently asked questions
How do you calculate the solvent mole fraction from a standard concentration?
The solvent's mole fraction is obtained by dividing the number of solvent moles by the total number of moles present in the solution (solvent plus solute). Our molar concentration calculator helps determine the number of solute moles from a concentration expressed in mol/L, a useful preliminary step for obtaining this mole fraction.
Why does this law only apply to a non-volatile solute?
If the solute is itself volatile (able to evaporate), it also contributes to the solution's total vapor pressure, making the calculation more complex than a simple proportionality to the solvent's mole fraction alone. This version of Raoult's law only covers the case of a non-volatile solute (dissolved salt or sugar, for example), where only the solvent evaporates significantly.
What is an 'ideal' solution in chemistry?
An ideal solution is a theoretical model where the attractive forces between solvent and solute molecules are comparable to the attractive forces between pure solvent molecules themselves, which makes Raoult's law exact. In practice, no real solution is perfectly ideal, but many dilute solutions come close enough for this formula to remain a good estimate.