What is partial pressure?
In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature.The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture.
It relies on the following isotherm relation:
Vx/Vtot =px /ptot =nx/ntot .
- Vx is the partial volume of any individual gas component (X)
- Vtot is the total volume of the gas mixture
- px is the partial pressure of gas X
- ptot is the total pressure of the gas mixture
- nx is the amount of substance of gas (X)
- ntot is the total amount of substance in gas mixture
The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids.
This general property of gases is also true in chemical reactions of gases in biology. For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood.