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Homework Help: Lots of people read this, but no one helped.

  1. Nov 29, 2007 #1
    1. The problem statement, all variables and given/known data

    (a) How many atoms of helium gas fill a balloon of diameter 29.4 cm at 23.5°C and 1.00 atm?
    (b) What is the average kinetic energy of the helium atoms?
    (c) What is the root-mean-square speed of the helium atoms?

    2. Relevant equations

    T=23.5 degrees celcius = 296.5K
    Avogadros #: 6.0221415 × 10^23

    3. The attempt at a solution
    (a)I dont know how to find the # of molecules. I know I need Avogadros #.
    (b)To solve this, dont I need the first one solved?
    (c) v=sqrt((3(8.31)(296.5K))/4.0026 g/mol) = 42.97m/s = 0.0429km/s (which is wrong coz I did this part first)
  2. jcsd
  3. Nov 29, 2007 #2


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    For a), you should use the ideal gas law, which is not in your list yet :smile:
    It gives you a connection between the pressure, volume, temperature and number of molecules, three of which are given.
    Then for b) you can apply one the first two formulas you quoted, plugging in the number N you found in a).
    And I don't understand the title.
  4. Nov 29, 2007 #3
    I had posted this question before, and it was read around 30 times and no one helped me out.
  5. Nov 29, 2007 #4
    Allright, so I did PV/RT=n, and this did not work. I tried multiplying my result times Avogadro's number and that was not right, so I have exhausted all of my tries, and I have still not arrived at the correct answer for (a). Any more advice?
  6. Nov 29, 2007 #5


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    The ideal gas law can also be written as
    [tex]p V = N k_B T,[/tex]
    where p is pressure, V is volume, N is number of molecules, T is temperature, and
    [tex]k_B \approx 1.38 \times10^{-23} \mathrm{J \cdot K^{-1}}[/tex]
    is the Boltzmann constant.

    This is much more useful here (though it is exactly the same, as there is the relation R = (Avogadro's #) * (Boltzmann constant)) and it should work. If not, you should give us more information.

    As for nobody responding, I apologize, but also note that people here helping other people is a courtesy and not a duty :smile: Maybe nobody knew the answer, maybe people were just busy. You may also want to consult this topic.
    By the way, I've asked questions on forums where about 300 people read the thread but nobody responded. So 30 is not that much :biggrin:
  7. Nov 29, 2007 #6


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    Please show details of your calculations. Just saying you didn't get the right answer isn't very helpful, because there are lots of small (or large) mistakes you could be making. We can't tell you what you're doing wrong if you don't show us what you did.
  8. Nov 29, 2007 #7

    Learning LaTeX would have made it easier to understand your calculations. Also, showing more of your work would have helped.

    You can refer to the following thread for a somewhat similar problem.



    Last edited: Nov 29, 2007
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