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Homework Help: Molar mass

  1. Mar 20, 2006 #1
    Hey everyone,
    I have this problem to solve:

    A hydrocarbon (CxHy) contains 92.3% of Carbon and 7.7% of Hydrogen.
    1) What is its molar mass, if the density of its smoke to the air is d=2.69 ?

    92.3/7.7 = 12
    So 12x=y
    This is the first equation. I have to find a second one to find what is x and what is y, in order to find the molar mass of CxHy.

    Is the density of water to the air can be written as d=(molar mass of the compound)/(molar mass of the water) ???

    Anyhelp/hints would be appreciated.
  2. jcsd
  3. Mar 20, 2006 #2
    Which are the units of this density, g/dm^3 ?
  4. Mar 20, 2006 #3


    User Avatar

    Staff: Mentor

    Sounds like it is ratio of its density (in gaseous form) to the air density.

    "Molar mass" of the air is about 0.78*28+0.21*32+0.01*40 (ie air has density as if it was gas of this molar mass). Hydrocarbon molar mass is 2.69 times higher.
  5. Mar 21, 2006 #4
    Got It!! Thanks Everyone :)
    Last edited: Mar 21, 2006
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