Partial Pressure Calculator for Dalton’s Law and Gas Mixtures
Calculate a gas component’s partial pressure, mole fraction or mixture total. You can also distribute total pressure across entered gas amounts, add mixed-unit component pressures or find an unentered remainder.
- Five focused calculation modes
- Exact decimal arithmetic
- Ten pressure units
- Numerical consistency checks
Online Partial Pressure and Mole Fraction Calculator
Select one relationship, enter absolute pressures or component amounts, and review the unrounded formula basis before using the result.
The main answer, composition checks, unit conversions and formula steps will appear here.
Partial Pressure Result Details
Conversions use the same unrounded result.
| Quantity | Relation | Unit or share | Value |
|---|
Calculation Steps
- Select a mode and enter valid values to see the calculation.
How to Use This Partial Pressure Calculator
- Choose the quantity or mixture calculation. Select partial pressure, mole fraction, total pressure, an amount-based mixture or a component-pressure mixture.
- Use absolute pressure. If your instrument reports gauge pressure, add the measured ambient pressure before entering the value.
- Enter complete numbers. Use decimal or scientific notation such as 2.095e-1 without commas, formulas or unit symbols.
- Select every input unit or composition basis. The calculator converts mixed pressure and amount units before applying the formula.
- List every intended component. Amount-based mixtures need a positive total gas amount. A zero row remains a valid zero-share component.
- Choose the result unit and precision. Rounding changes only the display, never another conversion or component result.
- Review the model limits. Confirm ideal-gas behavior, a common temperature and volume, no reaction, no condensation and no omitted component.
Most calculations take about two minutes when your composition and absolute-pressure data are ready.
Partial Pressure Formula and Dalton’s Law
Partial pressure is the pressure contribution of one constituent in a gas mixture. For an ideal mixture, IUPAC gives the component relationship as the amount fraction multiplied by total pressure.
Here, pi is the component partial pressure, xi is its amount fraction or mole fraction, and P is the total absolute pressure. The fractions of a complete mixture sum to 1, while the partial pressures sum to the total.
Five Supported Partial Pressure Calculations
| Mode | Inputs | Relationship |
|---|---|---|
| Partial pressure | Mole fraction and total pressure | pi = xiP |
| Mole fraction | Partial and total pressure | xi = pi/P |
| Total pressure | Partial pressure and mole fraction | P = pi/xi |
| Amount-based mixture | Component amounts and total pressure | xi = ni/Σn |
| Pressure-based mixture | Known component pressures, with optional known total | P = Σpi or pmissing = P - Σpknown |
Mole Fraction, Percent, ppm and ppb
Mole fraction is dimensionless. A value of 0.2095 equals 20.95%, 209,500 ppm or 209,500,000 ppb. The calculator keeps these as four display bases for the same composition.
For an ideal gas mixture at the same temperature and pressure, volume fraction equals mole fraction. This equivalence does not make mass fraction interchangeable with either value. IUPAC commonly uses amount fraction, and some technical texts use yi for a gaseous component.
Build a Gas Mixture from Component Amounts
When component amounts are known, add them before calculating any pressure share. The amount unit can differ by row because the calculator converts kmol, mol, mmol, µmol and nmol to one exact mole basis. A component entered as 500 mmol therefore contributes the same amount as 0.500 mol.
The listed amounts must describe the complete mixture. If a gas is omitted, the remaining rows are treated as though they still form 100% of the mixture, so every reported mole fraction and partial pressure will be too large. Enter a known zero component as zero rather than leaving its amount ambiguous.
A one-component basis is mathematically valid and returns x = 1. In a true multicomponent mixture, include at least two positive components. The calculator never rescales an entered percentage table because it uses component amounts directly. This avoids turning an incomplete composition into an apparently complete one.
Why the component pressures add back to the total
Each amount is divided by the same total amount, so the unrounded fractions sum to 1. Multiplying each unrounded fraction by P makes the unrounded partial pressures sum to P. Rounded table rows may appear a final digit above or below the displayed total. That is a presentation effect, not a new physical imbalance.
Result Checks, Rounding and Measurement Uncertainty
The result panel separates exact calculation logic from display precision. It parses each decimal as an exact ratio of integers, applies the chosen unit factor, performs the required multiplication, division, sum or subtraction, and rounds only when creating visible text. This preserves cases such as a very small fraction multiplied by a very large pressure and a small missing pressure found by subtracting two close large values.
Extra displayed digits do not increase experimental accuracy. If total pressure is measured to four significant figures and composition to three, report the final result at an appropriate precision for those inputs. Keep instrument calibration, resolution, repeatability and stated uncertainty with the result when the calculation supports laboratory or process work.
A calculation can be numerically exact for the entered decimals while the underlying model remains approximate. Ideal-mixture behavior, gas purity, temperature uniformity and phase equilibrium are separate scientific assumptions. Review those assumptions before treating a precise-looking value as accurate.
Worked Partial Pressure Examples
Oxygen at 20.95% of standard atmospheric pressure
Take xO2 = 0.2095 and P = 101.325 kPa. Multiplying the unrounded values gives:
Two-gas mixture from component amounts
A mixture contains 2.83 mol of O2 and 8.41 mol of N2O at 192 kPa. The oxygen fraction is 2.83/11.24, so its partial pressure is about 48.3416 kPa. The remaining component contributes about 143.658 kPa.
Gas collected over water
If a wet gas has a total pressure of 750 Torr and water vapor contributes 25.2 Torr, the dry gas contributes the unentered remainder.
This correction requires the vapor pressure at the actual temperature. Unequal liquid levels add a separate hydrostatic-pressure correction.
Partial Pressure Units and Conversions
Pressure ratios are unitless only after both pressures use compatible units. The calculator converts every input to pascals internally, then converts the exact result to your selected display unit.
| Unit | Pascal basis | Note |
|---|---|---|
| kPa | 1 kPa = 1,000 Pa | Common laboratory and engineering unit |
| bar | 1 bar = 100,000 Pa | Exact decimal relationship |
| atm | 1 atm = 101,325 Pa | Exact standard atmosphere |
| Torr | 1 Torr = 101,325/760 Pa | Exactly one 760th of an atmosphere |
| mmHg | 1 conventional mmHg = 133.322387415 Pa | Kept distinct from Torr |
| psi | 1 psi ≈ 6,894.757293168 Pa | Pound-force per square inch |
Absolute Pressure, Gauge Pressure and Wet Gases
Dalton’s law uses absolute pressure. A gauge reading measures pressure relative to local ambient pressure, so the required conversion is Pabsolute = Pgauge + Pambient. Do not assume ambient pressure is exactly 1 atm.
For a gas collected over a liquid, the measured total includes the liquid vapor. Enter the measured total and the verified vapor pressure in the missing-component option. The calculator does not estimate water vapor pressure from temperature, because such a lookup requires a defined dataset, range and equilibrium assumption.
Ideal-Mixture Assumptions and Real-Gas Limits
The equations assume a homogeneous gas mixture whose components share one temperature and volume, behave ideally and do not react. The model ignores condensation, dissociation, adsorption and intermolecular corrections.
Real-gas departures become more important at high pressure, low temperature and near phase boundaries. Consequential laboratory or process work may require measured composition, calibrated absolute-pressure data, fugacity coefficients or a validated equation of state.
- Use compatible definitions and absolute-pressure measurements.
- Include every gas and vapor that contributes to the measured total.
- Do not treat rounded displayed components as a new exact mixture.
- Verify uncertainty, instrument range and local safety procedures separately.
Common Partial Pressure Calculation Mistakes
- Entering 20.95 as a decimal fraction. Choose percent, or enter 0.2095 as the decimal fraction.
- Mixing gauge and absolute pressure. Dalton relationships require absolute pressure.
- Using mass percent directly. Convert each mass to moles before calculating mole fraction.
- Omitting water vapor. A wet-gas measurement includes the liquid vapor contribution.
- Assuming Torr and mmHg are identical. They are close, but this calculator keeps their stated conversion factors separate.
- Ignoring nonideal behavior. The ideal model may not represent a compressed or condensing gas accurately.
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Partial Pressure Calculator FAQs
What is the formula for partial pressure?
For an ideal gas mixture, a component’s partial pressure is its mole fraction multiplied by total absolute pressure: pi = xiP.
What does Dalton’s law of partial pressures state?
Dalton’s law states that the total pressure of an ideal, nonreacting gas mixture equals the sum of its component partial pressures: P = Σpi.
How do I calculate partial pressure from mole fraction?
Convert the composition to a decimal mole fraction, then multiply by total absolute pressure. For example, 20% means x = 0.20.
How do I find mole fraction from pressures?
Divide the component partial pressure by total absolute pressure: xi = pi/P. Use compatible pressure units before dividing.
How do I find total pressure from one component?
Divide component pressure by its positive mole fraction: P = pi/xi. A zero component pressure returns zero, while a zero fraction cannot identify a total pressure.
Is volume percent the same as mole percent?
For ideal gases compared at the same temperature and pressure, volume fraction equals mole fraction. Mass percent is different and requires molar-mass conversion.
How do I correct a gas collected over water?
Subtract the water-vapor partial pressure at the measurement temperature from the wet total pressure. Correct separately for unequal liquid levels when relevant.
Are Torr and mmHg exactly the same?
No. One Torr is exactly 1/760 of a standard atmosphere. Conventional mmHg uses a separate defined conversion, although the numerical difference is small.
Does partial pressure depend on the gas identity?
In the ideal-mixture model, the pressure share follows the component amount fraction. Real-gas interactions and phase behavior can introduce identity-dependent departures.
When should I avoid the ideal partial-pressure model?
Use a validated real-gas method or measured data near condensation, at high pressure, at low temperature, during reaction or whenever nonideal effects matter to the decision.
Formula, Terminology and Unit Sources
Disclaimer: This calculator provides educational and preliminary results. It does not replace calibrated measurements, validated thermodynamic models, laboratory procedures, uncertainty analysis, safety controls or qualified scientific review.