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General Chem trick question

Discussion in 'DAT Discussions' started by Kami, Jul 23, 2006.

  1. Kami

    Kami Member
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    The Osmotic pressure at STP of a solution made from 1 L of NaCl (aq) containing 117 g of NaCl is:

    a) 44.77 atm
    b) 44.87 atm
    c) 89.54 atm
    d) 117 atm

    Hint: the tricky part has to do with the moles of NaCl
     
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  3. sgdorfl90

    sgdorfl90 Senior Member
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    aqueous, so you get 2 moles out of 1 NaCl, so prolly C.
     
  4. thehipster

    thehipster Senior Member
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    Yeah, pretty straight forward, gotta be C
     
  5. mlle

    mlle Senior Member
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    what?! can someone please explain this in more detail... how do you approach this? thanks.
     
  6. R.L.H

    R.L.H LoVe mY liFE
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    Why 1 mole is 44.77 atm?
     
  7. Orthodox Bear

    Orthodox Bear Columbia Class of 2011
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    People, the solution is simple....
    Osmotic Pressure=MRTi
    THis can be manipulated to Osmotic Pressure=nRTi/V
    n= 117/58.5= 2 moles NaCl
    R*T at STP= 22.4
    i= 2, because NaCl breaks up into two ions
    Therefore Osmotic pressure= (2)(22.4)(2)= 89.6
     
  8. Kami

    Kami Member
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    Thats what I thought as 2 moles but it is actually 4 moles of NaCl and I am trying to figure it out why it is 4 moles. The rest is just plugging in the numbers to get the answer.


     
  9. tom_servo_dds

    tom_servo_dds Senior Member
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    It is 2 moles of NaCl, but NaCl completely dissociates in water so you get two moles of Na+ and 2 moles of Cl- total. If it didn't dissociate you would have:

    (2)(22.4)(1) = 44.8

    According to the equation OP = nRTi/V, but instead you get:

    (2)(22.4)(2) = 89.6
     
  10. thehipster

    thehipster Senior Member
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    Osmotic pressure, Boiling-Point Elevation, and Freezing-Point Depression all depend on Colligative Properties. So for this example, NaCl, each mole dissociates into 2 ions (Na+ and Cl-). If you got something that doesn't dissociate, like an organic compound (i.e. ethanol, C2H5OH) you only have the one mole. If you get a problem that deals with Colligative properties, you'll have to use this approach.
     

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