Gas Partial Pressure

  • In the natural world, gases rarely exist in pure form and are most often observed as a mixtures of gases. The concept of a "Partial Pressure" has been developed to quantify the concentration of a particular gas within a mixture of gases. The Partial Pressure of any particular gas within a mixture is defined as the pressure that the gas would exert in the absence of the other gases within the mixture. For example, atmospheric air is composed of oxygen, carbon dioxide, nitrogen, and water vapor. The partial pressure of oxygen in atmospheric air would be the pressure that the oxygen would exert if all of the other atmospheric gases were removed
  • A corollary to this concept is that summation of all the partial pressures of the gases in a mixture should yield the total pressure that the entire mixture exerts. Consequently, summing the individual partial pressures of all the gases that compose atmospheric air should yield 760 mm Hg, equivalent to atmospheric pressure.

Partial Pressures of Nitrogen and Oxygen in Air
Dry air is made of 78% nitrogen (N2) and 21% oxygen (O2). The partial pressures of these gases are simply the pressure they would exert if they were isolated in the same volume of space. Since atmospheric pressure is 760mm Hg, the 78% of molecules contributed of nitrogen exerts roughly 600 mm Hg of pressure while the 21% contributed by oxygen exert roughly 150 mm Hg of pressure.