About the Raoult's Law Calculator
This Raoult's law calculator finds the vapor pressure above an ideal solution in the two situations textbooks cover. In the first, a non-volatile solute such as sugar or salt is dissolved in a solvent: enter the masses and molar masses of both, the pure solvent's vapor pressure and the van 't Hoff factor, and the calculator gives the solvent mole fraction, the solution's vapor pressure and the vapor-pressure lowering. In the second, two volatile liquids are mixed: enter the mole fraction of one and the vapor pressures of both pure liquids to get the total pressure and the composition of the vapor.
It is useful for colligative-properties homework, for understanding why salt water evaporates more slowly than pure water, and for distillation problems where the vapor is richer in the more volatile component. The chart plots the liquid (bubble-point) line across all compositions so you can see where your mixture sits.
Raoult's law assumes an ideal solution — the two components attract each other about as strongly as they attract themselves. Mixtures like benzene–toluene or hexane–heptane follow it closely; strongly hydrogen-bonding or very different liquids deviate. Use any pressure unit, as long as all vapor pressures are in the same one.
With the default inputs, the vapor pressure of the solution is 23.34. Change any value above to recalculate instantly.
How to use the raoult's law calculator
- 1Choose a non-volatile solute or a mixture of two volatile liquids.
- 2Pick the pressure unit and enter the pure-component vapor pressure(s) at your temperature.
- 3For a solute, enter masses and molar masses of solute and solvent, plus i for electrolytes.
- 4For two liquids, enter the mole fraction of liquid A.
- 5Read the vapor pressure, then check the lowering or vapor composition.
Formula and method
Raoult's law states that each component of an ideal solution contributes a partial vapor pressure equal to its mole fraction in the liquid times the vapor pressure of the pure component. With a non-volatile solute only the solvent evaporates, so the solution's vapor pressure is x_solvent × P°. The solvent mole fraction counts every dissolved particle, so moles of solute are multiplied by the van 't Hoff factor i.
For two volatile liquids the partial pressures add (Dalton's law) to give the total pressure, which varies linearly with composition. The vapor composition follows from each partial pressure divided by the total, so the vapor is always enriched in the more volatile liquid.
- P°
- Vapor pressure of the pure liquid at the same temperature
- x
- Mole fraction in the liquid
- y
- Mole fraction in the vapor
- i
- Van 't Hoff factor (particles per formula unit)
Worked examples
18 g glucose in 100 g water at 25 °C
18 g ÷ 180.156 = 0.0999 mol glucose and 100 g ÷ 18.015 = 5.551 mol water, so x_water = 0.9823. P = 0.9823 × 23.76 = 23.34 mmHg, a lowering of 0.42 mmHg.
5.85 g NaCl in 100 g water (i = 2)
NaCl gives about two particles per formula unit: 0.1001 mol × 2 = 0.2002 mol of ions. x_water = 5.551 ÷ 5.751 = 0.9652, so P = 22.93 mmHg — roughly double the lowering of the same number of moles of glucose.
Equimolar benzene–toluene mixture
Benzene contributes 0.5 × 95.1 = 47.55 mmHg and toluene 0.5 × 28.4 = 14.2 mmHg, for a total of 61.75 mmHg. The vapor is 47.55 ÷ 61.75 = 77% benzene, much richer than the 50% liquid.
Frequently asked questions
What does Raoult's law say?+
Raoult's law says the partial vapor pressure of a component in an ideal solution equals its liquid mole fraction times the vapor pressure of the pure component: P_i = x_i·P°_i. Adding solute lowers the solvent's mole fraction and therefore its vapor pressure.
Why does a dissolved solute lower vapor pressure?+
Solute particles take up part of the liquid and lower the solvent mole fraction, so fewer solvent molecules are at the surface ready to escape. The effect depends on the number of particles, not their identity — a colligative property.
When does Raoult's law not apply?+
It fails for non-ideal solutions where unlike molecules attract more or less strongly than like ones. Acetone–chloroform shows negative deviation (lower pressure), and ethanol–water positive deviation, which leads to azeotropes.
What is the difference between Raoult's law and Henry's law?+
Raoult's law describes the solvent (or both components of an ideal mixture) and uses the pure-liquid vapor pressure. Henry's law describes a dilute dissolved gas or solute using an empirical Henry constant instead.
What is the vapor pressure of water at 25 °C?+
Pure water has a vapor pressure of about 23.76 mmHg (3.17 kPa) at 25 °C, rising to 760 mmHg at 100 °C. Always use the value at the temperature of your solution.