# What is the partial pressure of a gas?

Finds the partial pressure of one gas in a mixture with Dalton’s law: from its mole fraction and the total pressure, from its moles and the total moles, or from its moles, the volume and the temperature.

- Page: https://www.acalculator.org/chemistry/partial-pressure-calculator
- JSON spec: https://www.acalculator.org/chemistry/partial-pressure-calculator.json
- Version: 05ecf426ba66

## Default answer

Example with the default inputs (Work from Moles, Moles of this gas 2.83, Moles of all gases 11.24, Total pressure 1.89489267209474 atm): The partial pressure is 48.3416 kPa (0.477095 atm).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| from | Work from | The mole fraction and total pressure, the moles of the gas and of the whole mixture, or the moles, volume and temperature. |
| x | Mole fraction (X) | The gas’s share of all the moles in the mixture, from 0 to 1 (0.21 for oxygen in dry air). |
| n | Moles of this gas | The amount of the gas you want the partial pressure of, in moles. |
| nt | Moles of all gases | The total amount of gas in the mixture, this gas included, in moles. |
| P | Total pressure | The pressure of the whole mixture (absolute). |
| V | Volume | The volume of the container. |
| T | Temperature | The temperature of the gas; above absolute zero. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Partial pressure | The partial pressure, to 6 significant figures, in kPa and atm. |
| kpa | Partial pressure (kPa) | The partial pressure in kilopascals. |
| atm | Partial pressure (atm) | The partial pressure in standard atmospheres. |
| mmhg | Partial pressure (mmHg) | The partial pressure in millimeters of mercury. |
| fraction | Mole fraction | The gas’s share of the moles in the mixture (when known). |
| percent | Share of the mixture (%) | The mole fraction as a percent (when known). |

## Method

P_A = X_A × P_total, where X_A = n_A ÷ n_total. For one gas in a container: P_A = n_A × R × T ÷ V, with R = 8.31446261815324 J/(mol·K).

## Assumptions

- The gases are ideal and do not react with each other, so each gas acts as if it filled the container alone (Dalton’s law).
- Pressures are absolute; temperatures are absolute (K = °C + 273.15).

## Worked examples

1. from = moles, n = 2.83, nt = 11.24, P = 192,000 gives kpa = 48.341637, x = 0.251779. Source: OpenStax, Chemistry 2e, §9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions (Dalton’s law: P_A = X_A × P_Total), https://openstax.org/books/chemistry-2e/pages/9-3-stoichiometry-of-gaseous-substances-mixtures-and-reactions, Example 9.15 The Pressure of a Mixture of Gases: X(O₂) = 0.252, P(O₂) = 48.4 kPa.
2. from = gas, n = 0.0025, V = 0.01, T = 308.15 gives atm = 0.006321. Source: OpenStax, Chemistry 2e, §9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions (Dalton’s law: P_A = X_A × P_Total), https://openstax.org/books/chemistry-2e/pages/9-3-stoichiometry-of-gaseous-substances-mixtures-and-reactions, Example 9.14 The Pressure of a Mixture of Gases: P(H₂) = 6.32 × 10⁻³ atm.
3. from = fraction, x = 0.21, P = 101,325 gives atm = 0.21, kpa = 21.27825, result = 21.2783 kPa (0.21 atm). Source: OpenStax, Chemistry 2e, §9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions (Dalton’s law: P_A = X_A × P_Total), https://openstax.org/books/chemistry-2e/pages/9-3-stoichiometry-of-gaseous-substances-mixtures-and-reactions.

## FAQ

### What is partial pressure?

The pressure one gas in a mixture would exert if it filled the container alone. By Dalton’s law, the partial pressures of all the gases add up to the total pressure.

### How do I find partial pressure from the mole fraction?

Multiply: P_A = X_A × P_total. Oxygen is about 0.21 of dry air, so at 1 atm (101.325 kPa) its partial pressure is about 0.21 atm, or 21.28 kPa.

### How do I get the mole fraction?

Divide the moles of the gas by the moles of all gases: X_A = n_A ÷ n_total. 2.83 mol of oxygen with 8.41 mol of nitrous oxide gives 2.83 ÷ 11.24 = 0.252.

### Can I find partial pressure without the total pressure?

Yes, if you know the moles of the gas, the container volume and the temperature: P_A = n_A × R × T ÷ V, the ideal gas law for that gas alone.

### Do the partial pressures add up to the total?

Yes, for ideal gases that do not react. In the OpenStax example, 6.32 × 10⁻³, 2.53 × 10⁻³ and 7.58 × 10⁻⁴ atm add to 9.61 × 10⁻³ atm.

### Why does partial pressure matter?

Many processes depend on one gas, not the whole mixture: how much oxygen dissolves in blood, how fast a gas diffuses, or how much water vapor sits over a liquid all follow that gas’s partial pressure.

## Sources

- OpenStax, Chemistry 2e, section 9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions: Dalton’s law of partial pressures, P_A = X_A × P_Total, X_A = n_A ÷ n_Total; Examples 9.14 and 9.15. CC BY 4.0, retrieved 2026-10-03. https://openstax.org/books/chemistry-2e/pages/9-3-stoichiometry-of-gaseous-substances-mixtures-and-reactions
- NIST, CODATA value of the molar gas constant R = 8.314462618... J/(mol·K), exact. Retrieved 2026-10-03. https://physics.nist.gov/cgi-bin/cuu/Value?r