1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Calculate N2/O2 ratio at 3255m

  1. Jan 15, 2015 #1
    1. The problem statement, all variables and given/known data
    The ratio of nitrogen to oxygen at sea level is 4.0:1. Use Laplace's law of
    isothermal atmospheres to calculate the same ratio for 3255m above sea level
    Assume that T = 273 K

    2. Relevant equations
    [tex]P_fN2 = P_iN2 * e^-\Big({\frac{M_N2 g}{R T}}*h\Big)[/tex]
    [tex]P_fO2 = P_iO2 * e^-\Big({\frac{M_O2g}{R T}}*h\Big)[/tex]

    [tex]P_i = 1 atm[/tex]
    [tex]h = 3255 meters [/tex]
    [tex]M_N2 = 28 g/mol [/tex]
    [tex]M_O2 = 32 g/mol [/tex]
    T = 273K
    [tex]g = 9.8 m/s^2[/tex]
    R = 8.314 J/mol K

    3. The attempt at a solution
    I tried plugging in the numbers for both equations, but somehow that just makes my exponential go to 0 so I know that's not correct. I am wondering if I need to convert anything. I converted the initial pressure to Pa but that doesn't help me with an exponential of 0. For N2 I came up with approximately 393.52 for Mg/RT*h. e to -393.52 comes out zero on my calculator, I'm sure its not, because e to any power is never 0 but its too small for my calculator to read.
  2. jcsd
  3. Jan 15, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Your molecular weights must be expressed in kg / mole, rather than grams / mole.

    Also, since you are given the ratio of nitrogen : oxygen at sea level, then the partial pressure of each element will not be 1.0 atm. That's the pressure of the whole atmosphere at sea level.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted