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Homework Help: Chemistry problem involving gas laws

  1. Oct 8, 2008 #1
    A 2.80g sample of an unknown chlorofluorocarbon is decomposed and produces 552mL of chlorine gas at a pressure of 756 mmHg and a temperature of 298K.
    What is the percent chlorine (by mass) in the unknown chlorofluorocarbon?


    PV= nRT
    D= m/v


    Ok, so for this problem I tried using the PV=nRT (756*.552=n*62.364*298) equation to solve for n and I got 0.02245 mol. And from that, I figured the molar mass was 124.69 g/mol (having used the mass of 2.80 to derive this answer). And then I tried to set up an algebraic equation with variables to solve for the mass percent. I used the equation (1-x)(31.008) + 35.45x = 124.69. The (1-x) represents the Carbon and Fluorine components of the compound, and the 31.008 is their combined molar mass. The 35.45 is the molar mass of Clorine, and by solving this equation i got 21.09, but this was incorrect. So what should i do?
     
    Last edited: Oct 8, 2008
  2. jcsd
  3. Oct 8, 2008 #2

    Redbelly98

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    How many grams of chlorine are in 0.02245 mol of Cl2?

    Compare that to 2.80 g.
     
  4. Oct 8, 2008 #3
    but we don't know how many grams we have of chlorine...?
     
  5. Oct 8, 2008 #4

    Redbelly98

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    You know how many moles and how many grams/mole for chlorine. Use that.
     
  6. Oct 8, 2008 #5
    okay thank you i got it!
     
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