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Homework Help: Scuba Tank

  1. Nov 12, 2013 #1
    1. The problem statement, all variables and given/known data
    A scuba tank has a volume of 2800cm3. For very deep dives, the tank is lled with 50%
    (by volume) pure oxygen (O2) and 50% pure helium (He). The molar mass of helium
    is 4 g mol/1, and for oxygen is 32 g mol/1.
    How many molecules are there of each type in the tank if it is lled at 20 C to a
    total pressure of 10 atmospheres? [Correct answer: N(He) = N(O2) = 3:55  1023]

    2. Relevant equations

    3. The attempt at a solution
    Correct answer given: The number of He molecules and O2 molecules are the same, since they occupy 50% by volume,
    they have the same number of moles.
    Each gas will contribute 5 atm in the gas cylinder. Using pV = NkT,
    NO2 = NHe =3.51*10^23

    My question is, how can we deduce N and P for two kinds of gases are the same. Because according to PV=nRT, we can only deduce that P(He)/N(He)=P(O2)/V(O2)
  2. jcsd
  3. Nov 12, 2013 #2


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    Science Advisor
    Homework Helper

    The pressure P is the same because both gases are in the same tank. A tank has only one pressure, same for temperature. Since each gas occupies the same volume, the N's must also be the same.
  4. Nov 13, 2013 #3


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    Staff: Mentor

    Have you heard about Avogadro's hypothesis? If not - google.
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