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how much does air weigh?

Discussion in 'Basic Scuba Discussions' started by H2Andy, Feb 26, 2007.

  1. fisherdvm

    fisherdvm Solo Diver

    # of Dives: 200 - 499
    Location:
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    The ideal gas law is PV = nRT

    Lets assume g is the mass of the air we are calculating.
    M is the molecular weight of air = (0.8 x 28) + (0.2x32) = 28.8 moles / liter
    where 0.8 is the amount of nitrogen, and 0.2 is the amount of oxygen.
    n is the number of moles of air.
    Where R = 0.082, the gas constant.
    Substituting: Pressure = P = 1 atm
    Volume in liter = V
    Temperature in Kelvin (Temp in Celcius plus 273) = 298 (at 25 deg celcius)
    Mass of air per liter = M = 28.8 per liter at 25 degree celcius


    P x V = n x R x T = g / M x (R x T) or g = (M x P x V) / (R x T)

    Plugging them in, g will be 1.2 gram of air per liter at 1 atm, at 25 degree celcius, without any co2 or water vapor.

    Now if you want to add water vapor, carbon dioxide, etc... You have to calculate the molecular weight of these molecules, and fraction....

    Then you can adjust the pressure, to 3000 psi, by converting psi to ATM.

    Then you have to convert Fahrenheit to Celcius, etc...
     
  2. fisherdvm

    fisherdvm Solo Diver

    # of Dives: 200 - 499
    Location:
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    Based on the fact that 1 cu ft of air has 28.3168466 liters , my calculation for the weight of a cu ft of air is 33.98 grams.
     
  3. H2Andy

    H2Andy Blue Whale

    # of Dives: 200 - 499
    Location: NE Florida
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    which my amazing memory [1] tells me is about .07 lbs




    [1] by amazing memory i mean this site: http://www.easysurf.cc/cnver3.htm#gp2
     
  4. fisherdvm

    fisherdvm Solo Diver

    # of Dives: 200 - 499
    Location:
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    No, it is 0.074756 lbs.
     
  5. H2Andy

    H2Andy Blue Whale

    # of Dives: 200 - 499
    Location: NE Florida
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    at what temperature? and altitude?
     
  6. fisherdvm

    fisherdvm Solo Diver

    # of Dives: 200 - 499
    Location:
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    At 25 degree celcius and 1 atm or sea level ...

    And at a bad *** attitude....
     
  7. H2Andy

    H2Andy Blue Whale

    # of Dives: 200 - 499
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    ah, see my room was 23 degrees celcius
     
  8. Rick Murchison

    Rick Murchison Trusty Shellback Staff Member ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
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    Excuse me????
    I do believe you meant 28.8 grams/mole.
    At STP (physics) 1 mole of gas occupies 22.4 liters, or about .045 moles/liter.
    What's amazing is that in the end you managed to arrange the numbers to come up with close to the right answer!
    The "real" math using moles follows:
    .79X28 + .21X32 = 28.84 g/mole
    28.84g/mole / 22.4 liters/mole = 1.2875 g/liter
    1.2875 g/liter X 28.32 liters/CF = 36.462 g/CF
    36.462 g/CF / 454g/lb = .0803 lb/CF
    :D
    Rick
     
  9. mike_s

    mike_s Solo Diver

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    So does Nitrox weigh less?

    what about tri-mix? (it does have helium in it).
     
  10. frankc420

    frankc420 Loggerhead Turtle

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    Location: Central MS
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    :popcorn:
     

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