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Partial Pressure from Mole Fraction

Pi=xi PtotalP_i = x_i \, P_{\text{total}}
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Worked example: O2 in air: x = 0.209 at 1.00 atm → 21.177 kPa. Press Try an example to run it live, then adjust anything.

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Where Partial Pressure from Mole Fraction fits

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Dalton's Law of Partial PressuresHenry's Law (Gas Solubility)Gas Density from Molar MassCompressibility Factor (Z = PV/nRT)

See also

Dalton's Law of Partial Pressures: The additive statement itself, for when you have the components and want the total, or a wet gas to correct.Henry's Law (Gas Solubility): Both use partial pressure.Compressibility Factor (Z = PV/nRT): Both use total pressure.Gas Density from Molar Mass: Both use total pressure.Van der Waals Equation of State: Both use total pressure.Bernoulli's Equation (Two Points): Both use total pressure.
2 more relatedEquivalent Potential Temperature (Bolton): Both use total pressure.Pressure Head (h = P/ρg): Both use total pressure.
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Partial Pressure from Mole Fraction explained

PtotalxiPi

John Dalton realised in 1801 that gas molecules in a mixture ignore one another: each exerts the pressure it would exert alone, and the total is their sum. Divide through by the total and you get the form everyone actually uses: a component's partial pressure is its mole fraction times the total pressure. Dry air is 20.9% oxygen by mole, so at 101.325 kPa the oxygen partial pressure is 0.209 × 101.325 = 21.2 kPa, and that number, not the percentage, is what drives oxygen across the alveolar membrane.

This is why altitude matters and composition does not: the air on top of Everest is still 20.9% oxygen, but at 33 kPa total pressure the oxygen partial pressure falls to about 7 kPa, roughly a third of sea-level value. Divers meet the same arithmetic in reverse: at 40 m the ambient pressure is 5 atm, so ordinary air delivers an oxygen partial pressure above 1 atm and a nitrogen partial pressure near 4 atm, which is where oxygen toxicity and nitrogen narcosis begin. The trap is mixing up mole fraction with mass fraction or volume percent; only mole fraction goes into this equation, though for ideal gases volume percent happens to equal it.

Partial Pressure from Mole Fraction formula

Pi=xi PtotalP_i = x_i \, P_{\text{total}}
Where
  • PiP_i= Partial pressure of component i (kPa)
  • xix_i= Mole fraction of component i
  • PtotalP_{\text{total}}= Total pressure (kPa)
Revised 16 October 2026
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formula.expert. (2026, October 16). Partial Pressure from Mole Fraction Formula & Calculator. https://formula.expert/solvers/partial-pressure-mole-fraction

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