What is the partial pressure of a gas?
Find the partial pressure of one gas in a mixture with Dalton’s law. Type its moles and the total moles, or its mole fraction, with the total pressure, or type its moles, the volume and the temperature.
- Partial pressure
- 48.3416 kPa (0.477095 atm)
The partial pressure is 48.3416 kPa (0.477095 atm).
- Partial pressure (kPa)
- 48.3416
- Partial pressure (atm)
- 0.477095
- Partial pressure (mmHg)
- 362.592
- Mole fraction
- 0.251779
- Share of the mixture (%)
- 25.1779
Partial pressure: 48.3416 kPa (0.477095 atm). The partial pressure is 48.3416 kPa (0.477095 atm).
How to calculate
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.
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).
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).
- 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
Each example is checked against the calculator on every build.
- Work from Moles, Moles of this gas 2.83, Moles of all gases 11.24, Total pressure 1.895 atm gives Partial pressure (kPa) 48.341637, Mole fraction (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
- Work from Volume and T, Moles of this gas 0.0025, Volume 10 L, Temperature 95 °F gives Partial pressure (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
- Work from Mole fraction, Mole fraction (X) 0.21, Total pressure 1 atm gives Partial pressure (atm) 0.21, Partial pressure (kPa) 21.27825, Partial pressure 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
How it works
Pick how to work:
- Moles: X_A = n_A ÷ n_total, then P_A = X_A × P_total. The moles of the gas cannot be more than the moles of all gases.
- Mole fraction: P_A = X_A × P_total, with X_A from just above 0 to 1.
- Volume and T: P_A = n_A × R × T ÷ V, with R = N_A × k = 8.31446261815324 J/(mol·K), exact. No mole fraction is shown in this mode.
Unit sizes: 1 atm = 101,325 Pa; 1 kPa = 1,000 Pa; 1 bar = 100,000 Pa; 1 torr = 101,325 ÷ 760 Pa; 1 mmHg = 13.5951 × 9.80665 = 133.322387415 Pa; 1 psi = 4.4482216152605 ÷ 0.00064516 Pa. 1 L = 0.001 m³; 1 mL = 10⁻⁶ m³; 1 ft³ = 0.028316846592 m³; 1 gal = 0.003785411784 m³. K = °C + 273.15; K = (°F + 459.67) × 5/9.
Exact arithmetic. Each value is read as the exact decimal you typed, in its unit, and every unit size and R are exact, so each result is an exact fraction until it is rounded for display.
Output format. The answer line shows the partial pressure in kPa and atm, each to 6 significant figures with no thousands separators: 48.3416 kPa (0.477095 atm). The rows show kPa, atm, mmHg, the mole fraction and the percent, to 6 significant figures; values of 10¹⁵ or more, or below 10⁻⁶, show in scientific form.
When there is no answer.
- The moles of this gas are more than the moles of all gases.
- In the volume mode, a partial pressure over 10¹² Pa.
- A partial pressure too small to hold as a number.
Assumptions
- Ideal gases that do not react with each other.
- Pressures and temperatures are absolute.
Worked examples by hand
Oxygen and nitrous oxide (OpenStax Example 9.15). X(O₂) = 2.83 ÷ (2.83 + 8.41) = 2.83 ÷ 11.24 = 0.251779; P(O₂) = 0.251779 × 192 = 48.3416 kPa (the book: 0.252 and 48.4 kPa).
Hydrogen in a 10.0 L vessel (OpenStax Example 9.14). P = 0.00250 × 8.31446 × 308.15 ÷ 0.0100 = 640.530 Pa = 0.00632149 atm (the book: 6.32 × 10⁻³ atm).
Oxygen in air. P = 0.21 × 101.325 = 21.27825 kPa, shown as 21.2783 kPa (0.21 atm).
Other questions people ask
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.