Humid Air and the Ideal Gas Law

Pressure, temperature and volume for an ideal or perfect gas like air with water vapor - or moist air

The Ideal Gas Law express the relation between pressure, temperature and volume in an ideal or perfect gas.

The Ideal Gas Law

The Ideal Gas Law can be expressed with the Individual Gas Constant:

p V = m R T         (1)


p = absolute pressure (N/m2, lb/ft2)

V = volume of gas (m3, ft3)

m = mass of gas (kg, lb)

R = individual gas constant (J/kg oK, ft lb/slugs oR)

T = absolute temperature (oK, oR)

 Density can be expressed as

ρ = m / V         (1b)


ρ = density (kg/m3)

and equation (1) can be modified to

p = ρ R T         (1c)

The Individual and Universal Gas Constant

The individual gas constant can be expressed with the universal gas constant and the molecular weight of a gas like

R = Ru / Mgas         (2)


Mgas = molecular weight of the gas

Ru = 8314.47 = universal gas constant (J/(kmol K))

The Molecular weight and the Individual Gas Constants for air and water vapor are listed below:

Gas Individual Gas Constant - R Molecular Weight
Imperial Units
(ft lb/slug oR)
SI Units
(J/kg K)
Air 1716 286.9 28.97
Water vapor 2760 461.5 18.02

Air Pressure

Daltons Law states that

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

The total pressure in moist air can therefore be expressed as

pt = pa + pw         (3)


pt = total pressure (kPa)

pa = partial pressure dry air (kPa)

pw = partial pressure water vapor (kPa)

Dry Air Partial Pressure

By using (1) and (2), the dry air partial pressure can be expressed as

pa = ρa (286.9 J/kg K) T         (4)

Water Vapor Partial Pressure

The water vapor partial pressure can be expressed as

pw = ρw (461.5 J/kg K) T         (4b)

Unlike other gases in air, water vapor may condense under common conditions. Since the boiling point of water at normal atmospheric pressure (101.3 kPa) is 100oC, the vapor partial pressure of water is low compared to dry air partial pressure in moist air. Common values for vapor pressure in moist air is in the range 0.5 to 3.0 kPa.

Maximum vapor pressure before water vapor start to condense at an actual temperature is called saturation pressure - pws.

Related Topics

  • Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content

Related Documents

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