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Homework Help: Lab Final - finding molar mass of an unknown gas

  1. May 18, 2010 #1
    Its the end of the year in my AP chemistry class and one section of our lab final includes finding the molar mass of an unknown gas. we are given a balloon filled with the gas and we must conduct any tests to figure the molar mass out

    2. Relevant equations
    i believe the equation relating density and molecular weight is MW=drt/p

    3. The attempt at a solution
    heres what i've thought of
    measure the weight of the filled balloon (our scales go up to 2 or 3 decimals)
    fill a graduated cylinder with water, flip it upside down in a bucket of water and release the gas into the graduated cylinder to get the volume
    then dry the balloon and get the weight again
    use d=m/v to get the density and then use the above equation to calculate the MW

    however i have some problems with this strategy
    will the presence of gas effect the mass of the balloon (noting the sig figs of the scale)
    what temperature do i use for mw=drt/p, the temp of the water or room temp?

    if you have a diff strategy or any suggestions PLEASE let me know

    Last edited: May 18, 2010
  2. jcsd
  3. May 18, 2010 #2


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    Gold Member

    The presence of the gas should not affect the mass of the balloon. However, your strategy to determine the mass of the gas will fail unless you consider an additional factor. Hint: think Archimedes!

    Depending on how quickly you do the displacement measurement, it will have some value in between the two. You can minimize errors by letting the water equilibrate at room temperature.
  4. May 18, 2010 #3
    I forsee some problems with trying to weigh a balloon filled with a gas or releasing the gas into a graduated cylinder filled with water. If it is a small balloon, you could dunk it into a beaker of water inside of another beaker to approximate the volume of water it displaces. If you are crafty enough, you can measure the mass of the gas by evacuating a container, then releasing the contents of the gas into this, although this would be tough without the proper equipment.
    Last edited: May 18, 2010