Dalton's Law

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

      = Σ p_i                                 (1)

where

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)

where

p_i = pressure (Pa, psi)

n₁ = the number of moles of the gas

R = universal gas constant (J/(mol K), lb_f ft/(lb mol ^oR),  8.3145 (J/(mol K))

T = absolute temperature (K, ^oR)

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/molK) (300 K) / (0.005 m³)

= 997740 Pa

= 997 kPa

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