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Homework Help: Solving for molar mass

  1. Dec 15, 2007 #1
    [SOLVED] Solving for molar mass

    1. The problem statement, all variables and given/known data
    What is the molar mass of an ideal gas if a 0.622g sample of this gas occupies a volume of 300mL at 35*C and 789mm Hg?

    2. Relevant equations
    PV = nRT

    3. The attempt at a solution
    So n= no. moles present
    So I assumed n=.622/x

    so : x= .622gRT/PV
  2. jcsd
  3. Dec 15, 2007 #2

    Solve for big M.

    Make sure you convert all your units, R has the units:

    Last edited: Dec 15, 2007
  4. Dec 15, 2007 #3
    Ok so I am very confused here. How do I know how many moles I have? And do I need the entire PV= nRT formula?
  5. Dec 15, 2007 #4
    You do not need to know how many moles there are. I accidentally mistyped it, hopefully you see that I fixed it.

    Eventually, you will end up with the units g/mol, and you'll be fine from there.

    And yes, you need the entire equation, because R will allow you to attain the units moles and it will cancel out the other ones you do not need.
  6. Dec 15, 2007 #5
    is R not a constant? 0.08206 [tex]L\ atm\ mol^-^1\ K^-^1[/tex]

    so [tex]M=\frac {grams\ast R\ast T} {P\ast V}[/tex]

    convert 3mL to .300L and 789mm Hg to 1.038atm
  7. Dec 15, 2007 #6
    No R is a constant, I'm not sure what I said to imply it wasn't but it is.

    You're answer is correct, you get g/mol once you cancel out common units.
  8. Dec 15, 2007 #7
    Thank you very much!!!!
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