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Homework Help: Detemining the mass of water vapour in air (Ideal gas law)

  1. Mar 8, 2012 #1
    1. I am supposed to calculate the mass of water vapour in one cubic meter of air that has a relative humidity of 80% at 20°C.

    2. p[v] = (eM[w]) / (RT)

    p[v] = vapour density
    e = water vapour pressure
    M[w] = molar mass of water
    R = gas constant (8,314 J / (mol*K)
    T = temperature (293,15°K)

    3. I am struggling with in what unit the vapour density should be measured, as well as the pressure, Pascal? I know this problem is embarrassingly easy, but I've been trying to get the answer for several days, reading up on it online... I miss my old physics-book!

    Help would be sincerely appreciated!
  2. jcsd
  3. Mar 8, 2012 #2
    Probably it's best if you use SI units. You can take [itex] ρ = \frac{m}{V} [/itex] in [itex] \frac{kg}{m^3} [/itex] and pressure in Pascals.
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