Humid Air vs. the Ideal Gas Law
In a perfect or ideal gas the correlations between pressure, volume, temperature and quantity of gas can be expressed by the Ideal Gas Law.
The Universal Gas Constant, _{ Ru } is independent of the particular gas and is the same for all "perfect" gases, and is included in of The Ideal Gas Law:
p V = n R _{ u } T (1)
where
p = absolute pressure (N/m^{2}, lb/ft^{2})
V = volume (m ^{ 3, } ft^{3} )
n = is the number of moles of the gas present
R _{ u } = universal gas constant (J/mol ^{ o } K, lb_{f} ft/(lb mol ^{ o } R) = 8.3145 J/mol K = 0.08206 L atm/mol K = 62.37 torr /mol K
T = absolute temperature ( ^{ o } K, ^{ o } R)
For a given quantity of gas, both n and R _{ u } are constant, and Equation (1) can be modified to
p_{1} V_{1} / T_{1} = p_{2}V_{2}/ T_{2}(2)
expressing the relationship between different states for the given quantity of the gas.
Equation (1) can also be expressed as
p V = N k T (3)
N =number of molecules
k = Boltzmann constant = 1.38066 10 ^{ 23 } J/K = 8.617385 10 ^{ 5 } eV/K
 One mole of an ideal gas at STP occupies 22.4 liters.
The Ideal Gas Law express the relation between pressure, temperature and volume in an ideal or perfect gas.
The Ideal Gas Law expessed by the Induvidual Gas Constant
The Ideal Gas Law can be expressed with the Individual Gas Constant as
p V = m R T (4)
where
p = absolute pressure (N/m^{2}, lb/ft^{2})
V = volume of gas (m ^{ 3, } ft^{3} )
m = mass of gas (kg, slugs )
R = individual gas constant (J/kg ^{ o } K, ft lb/slugs ^{ o } R)
T = absolute temperature ( ^{ o } K, ^{ o } R)
Density can be expressed as
ρ = m / V (4b)
where
ρ = density (kg/m^{3}, slugs/ft^{3} )
and equation (4) can be modified to
p = ρ R T (4c)
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 = R _{ u } / M _{ gas } (2)
where
M _{ gas } = molecular weight of the gas
R _{ u } = universal gas constant ( 8314.47 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 ( kg/kmole ) 

Imperial Units ( ft lb/slug ^{ o } R ) 
SI Units ( J/kg K ) 

Air  1716  286.9  28.97 
Water vapor  2760  461.4  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
p _{ t } = p _{ a } + p _{ w } (3)
where
p _{ t } = total pressure (kPa)
p _{ a } = partial pressure dry air (kPa)
p _{ w } = partial pressure water vapor (kPa)
Dry Air Partial Pressure
By using (1) and (2) , the dry air partial pressure can be expressed as
p _{ a } = ρ _{ a } (286.9 J/kg K) T (4)
Water Vapor Partial Pressure
The water vapor partial pressure can be expressed as
p _{ w } = ρ _{ w } ( 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 100 ^{o}C, 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 are 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  p _{ ws } .
Related Topics
• Air Psychrometrics
Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Aircondition systems temperatures, absolute and relative humidities and moisture content in air.
• Basics
Basic engineering data. SIsystem, unit converters, physical constants, drawing scales and more.
Related Documents
Air  Composition and Molecular Weight
Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.
Air  Humidity Measurement from Dry and Wet Bulb Temperature
Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.
Air  Maximum Moisture Carrying Capacity
Maximum water content in humid air vs. temperature.
Air  Moisture Holding Capacity vs. Temperature
The moisture holding capacity of air increases with temperature.
Air  Molecular Weight and Composition
Dry air is a mixture of gases where the average molecular weight (or molar mass) can be calculated by adding the weight of each component.
Dry Air  Thermodynamic and Physical Properties
Thermodynamic properties of dry air  specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, Prandtl number, density and kinematic viscosity at temperatures ranging 175  1900 K.
Dry Air and Water Vapor  Density and Specific Volume vs. Temperature  Imperial Units
Density and specific volume of dry air and water vapor at temperatures ranging 225 to 900 degF (107 to 482 degC).
Moist Air  Daltons Law of Partial Pressure
The pressure in a mixture of dry air and water vapor  humid or moist air  can be estimated by using Daltons Law of partial pressures.
Moist Air  Degree of Saturation
Humidity ratio of moist air to humidity ratio of saturated moist air.
Moist Air  Density vs. Pressure
Density of moist air vs. pressure ranging 75  1000 mmHg.
Moist Air  Density vs. Water Content and Temperature
Density of the mix of dry air and water vapor  moist humid air.
Moist Air  Specific Volume
Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air
Moist Air  Specific vs. Relative Humidity
Specific humidity of moist air vs. relative humidity, water vapor and air density.
Moist Air  Water Vapor and Saturation Pressure
Saturation pressure of water vapor in moist air vs. temperature.
Nitrogen  Enthalpy, Internal Energy and Entropy vs. Temperature
Enthalpy, internal energy and entropy of Nitrogen as an ideal gas.
Nonideal gas  Van der Waal's Equation and Constants
The van der Waals constants for more than 200 gases used to correct for nonideal behavior of gases caused by intermolecular forces and the volume occupied by the gas particles.
The Ideal Gas Law
The relationship between volume, pressure, temperature and quantity of a gas, including definition of gas density.
Total and Partial Pressure  Dalton's Law of Partial Pressures
How to calculate total pressure and partial pressures for gas mixtures from Ideal Gas Law.
Universal and Individual Gas Constants
The Universal and Individual Gas Constants in fluid mechanics and thermodynamics. Individual gas constants for the most common gases.