Partial Pressure
Definition and meaning of Partial Pressure in chemistry.
Partial pressure is the pressure that a gas in a mixture would exert if it alone occupied the entire volume at the same temperature. In a gas mixture, the total pressure equals the sum of the partial pressures of all individual component gases, following Dalton's Law of Partial Pressures.
In more detail
Each gas in a mixture behaves independently, as though the other gases were absent, provided the gases do not react with one another. This allows chemists to predict gas behavior in mixtures and is essential for understanding gas reactions, chemical equilibrium, and physiological processes like respiration. Partial pressures are especially important when collecting gases over water, analyzing vapor pressure, or working with gas mixtures at varying temperatures and volumes.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing Law | Dalton's Law of Partial Pressures |
| Formula | Ptotal = P1 + P2 + P3 + ... (for n gases) |
| Common Units | atm, Pa, bar, torr, kPa |
In Earth's atmosphere at sea level (101.3 kPa total pressure), nitrogen contributes a partial pressure of approximately 79 kPa and oxygen contributes approximately 21 kPa, with the remaining roughly 1 kPa coming from argon and trace gases such as carbon dioxide. The partial pressures of all these component gases sum to equal the total atmospheric pressure of 101.3 kPa, in accordance with Dalton's Law.
Frequently asked questions
How do you calculate the partial pressure of a specific gas?
If you know the mole fraction of the gas and total pressure, use: Pgas = Xgas times Ptotal, where X is the mole fraction. Alternatively, for ideal gases: P = nRT/V.
Does Dalton's Law apply to all gas mixtures?
Yes, for ideal gases and most real gases at moderate pressures and temperatures. It breaks down at very high pressures where intermolecular forces become significant.