Raoult's Law Calculator
Estimate the vapor pressure of an ideal solution from the pure-solvent vapor pressure and either mole counts or solvent mole fraction.
Inputs
Live results
Pressure across solvent mole fraction
Reference values
| Solvent mole fraction | Solute mole fraction | Solution vapor pressure | Pressure lowering |
|---|
How to use the Raoult's law calculator
What this calculator does
This calculator estimates the vapor pressure contributed by a solvent in an ideal solution containing a non-volatile solute. It applies Raoult's law, p = x × p°, where p° is the vapor pressure of the pure solvent at the same temperature and x is the solvent mole fraction. It is useful for idealized chemistry work, teaching, laboratory planning, and checking hand calculations. It does not predict non-ideal activity coefficients, chemical reactions, temperature-dependent vapor-pressure data, or the total pressure of a mixture in which several components are volatile. The IUPAC definition of Raoult's law provides the formal thermodynamic context.
When to use it
Use the calculator when you need to estimate how a dissolved non-volatile material lowers a solvent's vapor pressure, compare formulations at the same temperature, prepare a classroom example involving mole fraction, or audit an experimental worksheet before using a more advanced non-ideal-solution model.
How to calculate
- The calculator opens with a complete demonstration: pure-solvent vapor pressure 24.7 mmHg, 0.5 mol solute, and 2 mol solvent. Its workbook is immediately available from Download Excel.
- Select Calculation method. Choose Use moles to derive mole fraction from mole counts, or Use solvent mole fraction when that dimensionless fraction is already known.
- Enter Pure-solvent vapor pressure and choose the matching Pressure unit. Changing the unit converts the current pressure rather than relabeling it.
- Complete the active composition fields. Results, the line chart, and the reference table update live.
- Read Solution vapor pressure as the main result, then use the secondary outputs to understand composition and vapor-pressure lowering. Download the current validated workbook as needed.
- Reset clears the demonstration and all calculated content. Download Excel then remains disabled until a complete valid state is entered again.
Input guide
Calculation method is required and controls how composition is supplied. Use moles expects two nonnegative decimal mole amounts; Use solvent mole fraction expects a decimal from 0 to 1. Pure-solvent vapor pressure is required, must be greater than zero, and accepts ordinary decimal notation such as 24.7; scientific notation and decimal commas are intentionally rejected to avoid ambiguity. It must correspond to the same temperature as the solution. The Pressure unit may be mmHg, kPa, atm, bar, or Pa. A higher pure-solvent pressure raises all predicted solution pressures proportionally. Moles of non-volatile solute is required in moles when the moles method is active; zero is valid and produces no lowering. Increasing solute moles while holding solvent moles fixed decreases the solvent fraction and solution pressure. Moles of solvent is required and must exceed zero; increasing it raises the solvent fraction. Solvent mole fraction is required only in fraction mode. A value of 1 represents pure solvent; 0 represents the limiting case with no solvent contribution. Do not enter a percentage such as 80 for 0.80.
Output guide
Solution vapor pressure is the predicted solvent contribution in the selected pressure unit. Solvent mole fraction and Solute mole fraction are dimensionless and sum to exactly 1 in this two-component model. Vapor-pressure lowering is p° – p; zero means pure solvent or zero solute. Relative lowering is the lowering divided by the pure-solvent pressure and equals the solute mole fraction for this ideal non-volatile-solute case. The summary pills repeat the active method, solvent fraction, and main pressure. The reference table lists solvent fraction, solute fraction, predicted pressure, and pressure lowering at five points. The chart plots the same model data and should be read as an exact ideal-law relationship, not an experimental fit.
Worked example
For the startup example, the solvent fraction is 2 ÷ (2 + 0.5) = 0.8000. Raoult's law gives 0.8000 × 24.7 mmHg = 19.76 mmHg. The pressure lowering is 24.7 – 19.76 = 4.94 mmHg, and the relative lowering is 20.00%. These values match the first-open cards, chart point, table row at x = 0.80 by interpolation, and exported workbook checkpoints.
Assumptions and interpretation
Raoult's law is exact only for an ideal solution in the thermodynamic sense. Real mixtures may show positive or negative deviations because unlike molecular interactions differ from like interactions. For practical work, use measured pure-component vapor pressure at the working temperature and compare predictions with experimental data. OpenStax's discussion of colligative properties and vapor-pressure lowering explains why the effect depends on particle fraction rather than chemical identity in the ideal limit.
Pressure units are converted internally through pascals, following the coherent SI pressure unit. The NIST overview of SI pressure units is helpful when moving among Pa, kPa, bar, atm, and mmHg. Unit conversion does not alter the physical state; it only changes how the same pressure is displayed.
Formula details and common mistakes
p_solution = x_solvent × p°_solvent
Δp = p°_solvent – p_solution
Common mistakes include using the solute fraction in place of the solvent fraction, mixing pressure units, entering the vapor pressure at a different temperature, treating a volatile solute as non-volatile, and assuming ideal behavior for strongly interacting components. Another frequent error is entering mass instead of moles. Mass must first be converted to amount of substance using molar mass. At the boundaries, a solvent fraction of 1 gives the pure-solvent pressure, while a fraction of 0 gives a zero solvent contribution in this simplified model.