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Dalton's Law

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The total pressure of a mixture of gases is made up by the sum of the partial pressures of the components in the mixture - also known from Gibbs'-Dalton's Law of Partial Pressures.

  • the total pressure exerted by a mixture of gases is the sum of the partial pressures of the individual gases

The total pressure in a mixture of gases can be expressed as:

p total = p1 + p2+ .. + p n

= Σp i (1)


p total = total pressure of mixture    (Pa, psi)

p i = partial pressure of individual gas   (Pa, psi)

Assuming that each gas behaves ideally - the partial pressure for each gas can calculated from the Ideal gas Law as

p i = n1 R T / V                     (2)


p i = pressure (Pa, psi)

n1 = the number of moles of the gas

R = universal gas constant (J/(mol K), lbf ft/(lb mol o R),  8.3145 (J/(mol K))

T = absolute temperature (K, o R)

V = volume (m3, ft3 )

Example - Partial Pressure of single Gas

If there is 2 moles of gas in 0.005 m3 volume ( 5 litre ) with temperature 27°C ( 300 K ) - the partial pressure of the gas can be calculated as

p i = (2) ( 8.3145 J/(mol K) ) (300 K) / (0.005 m3 )

= 997740 Pa

= 997 kPa

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