Partial Pressure Calculator
Calculate a gas component's partial pressure with Dalton's law, the ideal gas law, or either common form of Henry's law.
Inputs
Live result
Calculation detail
| Quantity | Meaning | Value | Unit |
|---|---|---|---|
| Total pressure | Pressure of the complete gas mixture | 1.000 | atm |
| Mole fraction | Selected gas moles divided by total moles | 0.210 | dimensionless |
| Partial pressure | Pressure attributable to the selected gas | 0.210 | atm |
How to use the partial pressure calculator
What this calculator does
This calculator finds the pressure attributable to one gas in a mixture. It supports four standard routes: Dalton's law from total pressure and mole fraction; the ideal gas law from moles, absolute temperature, and volume; and two Henry's law conventions for a gas dissolved in a liquid. The result is a thermodynamic estimate based on the selected model. It does not determine whether a gas mixture is chemically reactive, whether a real gas strongly departs from ideal behavior, or whether a chosen Henry's constant is valid at your exact temperature and solvent composition.
When to use it
Use it to check the oxygen contribution in a breathing-gas mixture, convert a gas composition into pressure for a laboratory exercise, estimate the pressure a known amount of ideal gas would exert in a vessel, or relate dissolved-gas concentration to equilibrium gas pressure. For the underlying mixture relationship, see the University of Central Florida's explanation of gas mixtures and Dalton's law.
How to calculate
- The calculator opens with a complete demonstration: Dalton's law, total pressure 1.000 atm, and mole fraction 0.210. The corresponding example workbook is already checked and Download Excel is immediately available.
- Select the Calculation method that matches your data. The visible fields change automatically.
- Replace the demonstration values with your measurements. Use decimal points, not decimal commas, and do not enter scientific notation or unit symbols inside the field.
- Read Partial pressure, the converted Pressure in kPa, and the equation shown below the cards. The detail table repeats each active input and the result.
- Select Download Excel to export the current validated model. Select Reset to clear all data; after reset, export stays unavailable until a complete valid set is entered again.
Input guide
Calculation method is required and chooses the formula. Total pressure is a positive decimal in atmospheres; 1.000 atm is a realistic example. A larger total pressure increases partial pressure in direct proportion. Mole fraction is required for Dalton's law, must be from 0 through 1, and is entered as a fraction such as 0.210 rather than 21. A frequent mistake is typing a percentage without converting it to a fraction.
For the ideal-gas route, Amount of gas is a nonnegative decimal in moles, Temperature is an absolute temperature above 0 K, and Mixture volume is a positive decimal in liters. Example values are 0.50 mol, 298.15 K, and 12.0 L. More moles or a higher temperature raises pressure; more volume lowers it. Never enter Celsius directly where kelvin is requested. The model uses R = 0.082057366 L·atm·mol⁻¹·K⁻¹, consistent with the standard ideal-gas relation described in this ideal gas law reference.
For Henry's law, Henry's law constant is required, positive, and must match the selected convention and the experimental temperature. In the concentration method its unit is L·atm/mol and it multiplies Dissolved-gas concentration in mol/L. In the mole-fraction method its unit is atm and it multiplies Dissolved-gas mole fraction, which must lie from 0 through 1. Because several reciprocal Henry constants exist in chemistry, copying a number without its definition and units is a common and potentially very large error.
Output guide
Partial pressure is the principal result in atmospheres. A zero result is valid only when the relevant amount, concentration, or mole fraction is zero. Pressure in kPa is the same physical pressure converted with 1 atm = 101.325 kPa. Method identifies the active law, while the equation line shows the exact substitution used. The summary pills repeat the selected method, current pressure, and export readiness. In the detail table, Quantity names each model field, Meaning gives its role, Value shows the current number, and Unit preserves dimensional context. These are calculated identities or model estimates, not safety recommendations.
Worked example
The first-open example represents a gas whose mole fraction is 0.210 in a mixture at 1.000 atm total pressure. Dalton's law gives pᵢ = P × xᵢ = 1.000 × 0.210 = 0.210 atm. Multiplying by 101.325 converts this to 21.278 kPa at the displayed precision. The startup cards, table, live announcement, and workbook all use these same values.
Formulas and interpretation
Dalton: pᵢ = Ptotal × xᵢ | Ideal gas: pᵢ = nᵢRT / V | Henry: p = Kc or p = Kx
Dalton's law treats each ideal-gas component as contributing independently to the total pressure. The ideal-gas route is algebraically consistent with Dalton's law when all components share the same temperature and volume. Henry's law addresses equilibrium between a dissolved gas and the gas phase, so its constant depends on the gas, solvent, temperature, and convention. Chemistry LibreTexts provides a fuller discussion of Dalton's law of partial pressures.
Limits and common mistakes
Real gases can deviate from ideal behavior at high pressure, low temperature, or when intermolecular forces are important. Henry's law is most dependable in dilute solutions and over conditions where the chosen constant was measured. Keep units paired with the formula, use kelvin for gas-law temperature, and distinguish a mole fraction such as 0.21 from a percentage such as 21%. For pressure-unit definitions and traceability, consult NIST's SI guidance for pressure.