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Homework Help: 3 Gases On a Graph, Which is True ?

  1. Oct 24, 2009 #1
    3 Gases On a Graph, Which is True ??

    1. The problem statement, all variables and given/known data

    http://img103.imageshack.us/img103/9302/spdistus8.jpg [Broken]

    Based on the distributions show above, which statements for the three gasses I II and III are true?
    Note that the temperature is constant for all three.

    a) Gas III is the heaviest of the three gasses.

    b) The most probable speed of the molecules of gas II is greater than that of the molecules of gas I.

    c) All the molecules of gas III have an actual speed that is greater than those of gas I.

    d) The average kinetic energies of the three gasses would be different.

    e) Increasing the temperature would increase the most probable speed of the molecules of each gas.

    2. Relevant equations

    Knowing About Gases.

    3. The attempt at a solution

    For A: I got False because on a graph My Professor showed in class he had the example of Curve I as O2 and Curve III as He.. and obviously He is the smallest.

    For B: The most probable speed is at the peak so obviously Molecule II isn't as high as molecule I so that is False.

    For C: All of Gas III Speed Are NOT always greater then the speeds of Gas I so False

    For D: I got True because the Formula makes them all dependent on their mass. *EDIT* This is True Because the Temp is Constant.

    For E: I got False, because when you increase the temp is doesn't move the peak of the curves.

    I need Help, Please :)

    Thank You.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 24, 2009 #2


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    Re: 3 Gases On a Graph, Which is True ??

    "Because my prof said so" is not a good approach to learning science, and neither is, "because it was true for these specific examples, it must be true in general." If you made an effort to determine the relevant equations beyond just "knowing about gases", you might find out what is the *general* mathematical relationship between the masses of the gas particles and the widths of these velocity distributions.

    Be careful. Which axis represents speed, the horizontal or the vertical? Is it the height of the peak that is relevant here, or its location?

    Isn't that a contradiction? If the temperature of the gas is proportional to its avg. kinetic energy, and all three gases have the same temperature, then how can statement D, which claims that that the three gases have different avg. kinetic energies, be true?

    How do you know that? Again, some actual physics, such as a relation between the temperature, particle mass, and peak/width of velocity distribution, is needed here.
  4. Oct 24, 2009 #3
    Re: 3 Gases On a Graph, Which is True ??

    A) Heavier Gases Move Slower, so that should still make this False
    B) Is True like you said, because the height is irrelevant they ask for the speed which is along the horizontal axis.
    C) False
    D) That was a typo, Should be False Since like I stated the Temp is constant which wouldn't make the Kinetic Energy Differ.
    E) True because "As Temp increases, the most probable speed increases, and the # of molecules moving very quickly increases."

    Hows this sound?
  5. Oct 24, 2009 #4


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    Re: 3 Gases On a Graph, Which is True ??

    Yeah, it sounds better to me. Another way to think of part a is that if the temperature of all three gases is is the same, then the avg. kinetic energy is the same. The avg speed definitely corresponds to the centre of the distribution (the peak value). If the avg. speeds of the gases are different, then their particle masses must be different too, since this is the only way for the kinetic energies to be the same. The ones with the higher speeds must have lower masses in order for the KE to remain the same, and vice versa.
  6. Oct 24, 2009 #5
    Re: 3 Gases On a Graph, Which is True ??

    Thanks a lot your tips were very helpful in helping find what i needed in my textbook to answer this question.

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