Raoult's Law Calculator — Vapor Pressure of Ideal Solutions (Metric & US Units)

Raoult's Law Calculator — Vapor Pressure of Ideal Solutions

Our Raoult's Law calculator is a convenient tool to calculate the vapor pressure of an ideal solution. It provides calculation options for the vapor pressure of a solution, the vapor pressure of the pure solvent, and the mole fraction of the solute or solvent.

Raoult's Law Equation

Raoult's law states that the relative lowering of the vapor pressure of a solution containing a non-volatile solute is equal to the mole fraction of the solute in the solution. Mathematically:

p_solution = x_solvent × p°_solvent

Where:

• p_solution — vapor pressure of the ideal solution
• x_solvent — mole fraction of the solvent in the solution
• p°_solvent — vapor pressure of the pure solvent

The mole fraction of the solvent is calculated as:

x_solvent = n_solvent / (n_solute + n_solvent)

How to Use the Raoult's Law Calculator

The calculator offers two modes:

Mode 1: Using Moles

1. Enter the moles of solute (n_solute, in mol)
2. Enter the moles of solvent (n_solvent, in mol)
3. Enter the vapor pressure of the pure solvent (p°) and select the pressure unit (mmHg, Pa, kPa, hPa, atm, bar, psi, or inHg)
4. Click Calculate — the calculator will display the mole fractions and vapor pressure of the solution in all supported units

Mode 2: Using Mole Fraction

1. Select what to solve for: vapor pressure of solution, vapor pressure of the pure solvent, or mole fraction of the solvent
2. Enter the two known values with their units
3. Click Calculate — the result is shown together with a complete pressure conversion table

Raoult's Law Example

Suppose you dissolve 1 mol of sugar (non-volatile solute) in 4 mol of water (solvent). The vapor pressure of pure water at 20 °C is 17.54 mmHg. Using Raoult's Law:

• x_solvent = 4 / (1 + 4) = 0.8000
• p_solution = 0.8000 × 17.54 mmHg = 14.032 mmHg
• Vapor pressure lowering: 17.54 − 14.032 = 3.508 mmHg (20.0%)

What Is Raoult's Law Used For?

Raoult's law applies to solutions containing non-volatile solutes dissolved in a volatile solvent, which form ideal solutions. Common applications include:

• Colligative properties — vapor pressure lowering, boiling point elevation, freezing point depression, osmotic pressure
• Industrial distillation — predicting the behavior of liquid mixtures
• Food science — calculating water activity in food products
• Pharmaceutical chemistry — understanding solvent behavior in drug formulations
• Environmental chemistry — studying evaporation of pollutants from water

Pressure Units — Metric and American Systems

The calculator supports both the metric (SI) and American measurement systems:

• Metric: Pascal (Pa), kilopascal (kPa), hectopascal (hPa), atmosphere (atm), millimeters of mercury (mmHg / Torr), bar
• American: pounds per square inch (psi), inches of mercury (inHg)

Results are always displayed in all units simultaneously for easy reference.

Raoult's Law Graph

The Raoult's Law graph shows vapor pressure (y-axis) versus mole fraction (x-axis). For an ideal solution, this is a straight line — the partial vapor pressure of a substance is directly proportional to its mole fraction when temperature is held constant. Positive deviations from Raoult's law indicate stronger solvent-solvent interactions than solute-solvent interactions; negative deviations indicate the opposite.

How to Calculate the Mole Fraction Using Raoult's Law

The mole fraction of the solvent can be found by rearranging the Raoult's law formula:

x_solvent = p_solution / p°_solvent

Divide the vapor pressure of the solution by the vapor pressure of the pure solvent. The mole fraction of the solute is then x_solute = 1 − x_solvent.

How to Find the Vapor Pressure of the Pure Solvent

If you know the vapor pressure of the solution and the mole fraction of the solvent, you can solve for the vapor pressure of the pure solvent:

p°_solvent = p_solution / x_solvent

FAQs

Does Raoult's Law apply to all solutions?

No. Raoult's Law is exact only for ideal solutions where solute-solvent interactions are the same as pure component interactions. Real solutions show positive deviations (interactions weaker than ideal) or negative deviations (interactions stronger than ideal).

What is a non-volatile solute?

A non-volatile solute has a negligible vapor pressure compared to the solvent. Examples include salts (NaCl), sugars (glucose, sucrose), and ionic compounds. When dissolved, they do not contribute to the vapor pressure of the solution.

What is the difference between Raoult's Law and Henry's Law?

Raoult's Law applies to the solvent in dilute solutions and across the full concentration range for ideal solutions. Henry's Law applies to the solute in dilute solutions. At infinite dilution, the solute obeys Henry's Law while the solvent obeys Raoult's Law.

What pressure units does the calculator support?

The calculator supports Pa, kPa, hPa, atm, mmHg (Torr), bar (metric) and psi, inHg (American/US customary). Results are shown in all eight units simultaneously.