Vapor Pressure Calculator
Estimate a new equilibrium vapor pressure with the Clausius – Clapeyron relation and evaluate ideal-solution pressure with Raoult's law.
Clausius – Clapeyron inputs
Raoult's law inputs
Live results
Pure solvent vs. ideal solution
Calculation details
| Method | Input / reference | Calculated result | Interpretation |
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How to use this vapor pressure calculator
What this calculator does
This calculator handles two related equilibrium-pressure tasks. The Clausius – Clapeyron section estimates how the vapor pressure of a pure substance changes between two absolute temperatures when one pressure and the molar enthalpy of vaporization are known. The Raoult's law section estimates the vapor pressure of an ideal solution from the pure solvent pressure and the solvent mole fraction. These are model-based estimates, not laboratory measurements, and their accuracy depends on how closely the substance or solution follows the assumptions.
When to use it
Use it to check a chemistry homework calculation, compare vapor pressure at two temperatures, estimate how adding a nonvolatile solute lowers a solvent's vapor pressure, or create a reproducible workbook for a lab note or engineering calculation. The calculator is especially useful when you need transparent intermediate values rather than only one final number.
How to calculate
- The page opens with a complete demonstration using water-like values, so the results and the Excel workbook are immediately available.
- Replace Initial temperature and Final temperature with positive absolute temperatures in kelvins.
- Enter Initial pressure, choose its pressure unit, and provide Molar enthalpy of vaporization in kJ/mol.
- For the ideal-solution estimate, enter the Mole fraction of the solvent from 0 to 1 and the Vapor pressure of the solvent with its unit.
- Read the live results and comparison chart, then select Download Excel to export the current canonical values. Reset clears the demonstration and disables export until a complete valid state is entered again.
The underlying temperature – pressure relationship is the integrated Clausius – Clapeyron equation. The Chemistry LibreTexts explanation of the Clausius – Clapeyron equation provides useful derivation and context.
Input guide
Initial temperature and Final temperature are required positive decimal values in kelvins; 280 K and 263 K are realistic examples. A common mistake is entering Celsius directly. Increasing the final temperature normally raises the calculated final pressure. Initial pressure is required and must be positive; the selected unit applies to both the input and final-pressure result. Molar enthalpy of vaporization is required in kJ/mol; 40.66 kJ/mol is used in the demonstration. Larger enthalpy magnitudes make vapor pressure more temperature-sensitive. Mole fraction of the solvent is required, dimensionless, and restricted to 0 through 1; 0.98 means 98% of the liquid-phase moles are solvent. Vapor pressure of the solvent is required and positive; 47.1 torr is the demonstration value. Do not enter a percentage such as 98 for a mole fraction of 0.98.
Output guide
Final pressure is the Clausius – Clapeyron estimate in the selected pressure unit. Pressure change is final minus initial, so it may be negative when temperature falls. Pressure ratio is P₂/P₁ and is dimensionless. Temperature change is T₂ – T₁ in kelvins. Solution vapor pressure is the Raoult's law result in the chosen unit. Solute lowering is the pure solvent pressure minus the solution pressure, and Solution reduction expresses that lowering as a percentage of the pure solvent pressure. The chart compares the two Raoult pressures on a zero-based common scale.
Worked example
With T₁ = 280 K, T₂ = 263 K, P₁ = 102,325 Pa, and ΔH = 40.66 kJ/mol, the exponent is – 40,660/8.314462618 × (1/263 – 1/280). Multiplying P₁ by the resulting exponential factor gives about 33,089.65 Pa. For Raoult's law, multiplying 47.1 torr by a solvent mole fraction of 0.98 gives 46.158 torr, displayed as 46.16 torr, and a lowering of 0.942 torr, displayed as 0.94 torr.
Model assumptions and interpretation
The integrated Clausius – Clapeyron form treats the vapor as ideal, neglects the condensed-phase molar volume compared with the vapor volume, and assumes the enthalpy of vaporization is constant across the interval. Over a narrow temperature range, this is often useful; over a broad range or near a critical point, a substance-specific correlation or experimental data is preferable. The NIST Chemistry WebBook is an authoritative source for experimental thermochemical and phase-change data, including vapor-pressure correlations for many compounds.
Raoult's law is most accurate for ideal or nearly ideal liquid mixtures. Strong specific interactions, association, dissociation, or highly dissimilar molecular species can produce positive or negative deviations. For multicomponent vapor mixtures, total pressure is typically interpreted together with partial pressures; the LibreTexts treatment of Raoult's law explains the ideal-solution relationship in more detail.
Pressure units cancel in the Clausius – Clapeyron ratio as long as P₁ and P₂ use the same unit. Temperatures do not cancel and must be absolute. The gas constant used here is 8.314462618 J/(mol·K). For safety or process design, treat the result as a screening estimate and verify it against substance-specific data and applicable engineering standards.