This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood consequently, mixture ratios, like that of breathable 20% oxygen and 80% Nitrogen, are determined by volume instead of by weight or mass. 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 general property of gases is also true in chemical reactions of gases in biology. Gases dissolve, diffuse, and react according to their partial pressures but not according to their concentrations in gas mixtures or liquids. The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture ( Dalton's Law). In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. Pressure of a component gas in a mixture The atmospheric pressure is roughly equal to the sum of partial pressures of constituent gases – oxygen, nitrogen, argon, water vapor, carbon dioxide, etc.
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