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Air Density

  1. Aug 31, 2011 #1
    Hello All,

    I am trying to calculate the density of moist air. Am i correct in saying.......

    density of dry air = 1.2754 kg/m^3

    Now i can convert this to mmol/m^3 and

    density of dry air = 4.384692933394098e+04 mmol/m^3
    at a given point in time
    density of CO2 in air = 16 mmol/m^3
    density of h2o = 500 mmol/m^3

    Am i correct in saying......

    density of moist air = (density of dry air)+(density of CO2)+(density of H2o)

    Any help at all is much appreciated.
  2. jcsd
  3. Aug 31, 2011 #2


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    I think it really depends on the humidity of the air. Different amounts of moisture will cause the density of the air to vary. Also, why are you adding in CO2? Isn't that already included in dry air?
  4. Sep 1, 2011 #3
    The density of humid air is a function of the air temperature, total air pressure, and the vapor pressure. This density is a simple product of the molecular number density (n) per cubic meter and the mean molecular mass (m). That is, d=nm.

    The number density can be calculated using the formula n = P/kT, where P is the pressure in Pascals, k is Boltzmann's Constant, and T is the temperature in Kelvins.

    The standard mass of a dry air molecule is 48.0985 x 10^-27 kg.
    The standard mass of a water vapor molecule is 29.9150512 x 10^-27 kg. This is the VSMOW standard.

    Calculate n separately for dry air and water vapor and do the simple maths.
  5. Sep 1, 2011 #4


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    There will be a different answer, depending on whether
    1. you start with a given mass of air and keep it constant - then add some extra mass in the form of water vapour or,
    2. You keep the pressure constant and consider the same total mass as before.

    It is important to remember that air is not 'some sort of sponge' which can 'soak up' water vapour. You just have two gases, each with its own partial pressure. Warm air can contain more water vapour simply because the vapour pressure of the water is higher.

    It is interesting to note that humid air is less dense than dry air (same conditions) because the molecular mass of H2O is less than that of O2 or N2. That is a bit counter-intuitive, I think.
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