Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Calculating maximum amount of water vapor per unit volume

  1. Sep 17, 2011 #1
    1. The problem statement, all variables and given/known data

    Compute the maximum amount of water vapor per unit volume that air can hold at the surface, where Ts = 288 K, and at a height of 10 km where T = 220 K. Express your answers in kg m-3.

    2. Relevant equations

    [itex]e_{s}=Ae^{\beta T}[/itex]

    [itex]e=\rho _{v}R_{v}T[/itex]

    3. The attempt at a solution

    Since saturation occurs when e=[itex]e_{s}[/itex], I figured I would set the two equations equal to each other. However, solving for [itex]\rho _{v}[/itex] doesn't work... The units don't work out, and I get a really large number... I feel like I have to somehow relate this to the total pressure of the air, but I'm unsure how to go about this.
  2. jcsd
  3. Sep 19, 2011 #2
    I was incorrect in my previous post saying that the units didn't work out... Solving for [itex]{\rho _{v}}[/itex] when e=[itex]e_{s}[/itex] does produce an answer in [itex]\frac{kg}{m^{3}}[/itex].... However I'm getting 1,103,248.397 [itex]\frac{kg}{m^{3}}[/itex], for the first case where T=288 K, which is way off from what I should be getting (0.0126 [itex]\frac{kg}{m^{3}}[/itex]).

    I believe I then have to use the ideal gas equation, pV=nRT, plugging in p for e... But this is where the confusion comes in. Hopefully someone can help me with this tonight, since this HW is due tomorrow morning...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook