First described by John Dalton (1766-1844) in 1801, this gas law is also called 'Dalton's law of partial pressure'. It states that:

'The total pressure exerted by a gaseous mixture is equal to the sum of the pressures that would be exerted by the gases if they alone were present and occupied the total volume.'

Ptot = A + p2 + ... + pn where: p1, p2, ... pn represent the partial pressures of each component.

Each gas in a mixture acts as if the other gas was not present, the pressures that come from each gas can simply be added. Dalton's law allows calculating the partial pressure of each gas as follows:

'The partial pressure of a gas (p¡) equals the product of total pressure of the gaseous mixture (Ptot) and the fraction of the gas (Fj)'

where: Fraction (F) is defined as a part of 1; i. e. in air FO2 is 0.21.

Practical relevance: Gases which are non toxic when inhaled at ambient pressure in a certain percentage of a gaseous mixture (Vol. %) may become toxic when inhaled at elevated total pressure because the partial pressure, and not the percentage in a gaseous mixture, causes toxicity.

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