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Homework Help: Oxygen pressure

  1. Jun 26, 2011 #1
    In a spherical vessel with a hydrogen H2 and oxygen mixture O2 it is stated in a problem that we can derive that the concentration of oxygen [O2] can be derived to be:

    P / 3RT. With P the total pressure, R the gas constant en T temperature?

    How on earth is this possible? I thought the ideal gas law stated that p = n R T (with n the concentration per unit volume). Any help?
  2. jcsd
  3. Jun 27, 2011 #2


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    It would depend on how you define 'n'. It is usually defined as total moles of gas. In this case it is a mixture of H2 and O2. If we let n = moles of oxygen AND we assume that the mixture is stoichiometric (ie. 2H2 + O2) you get the result you posted.

    See if you can rearrange the general equation, n = PV/RT into a concentration (moles per liter), concentration = P/3RT. If the general gas law only defines the system for all gases, where does that '3' come from?
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