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Calculate osmolality of a 0.1% and a 0.9% NaCl.?

  1. Mar 20, 2012 #1

    Wek

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    I tried to calculate the osmolality (mOsm/kg) of a 0.1% and a 0.9% NaCl solution but I am not sure I'm doing this right.

    First I calculated the NaCl grams in a liter of solution, so for 0.1% I have 0.01g of NaCl. Then I divided by the 58.44 (MW) to get moles. Then I divided the moles by 1 liter (since the osmolality is per 1 kg of solution) to get the molality. Then I multiplied the molality by the dissociation constant of NaCl (1.8). Then I divided by 1000 to get mOsm.

    The final answer I got is 0.308008214 mOsm/kg. Can someone double check my math?

    Thanks
     
  2. jcsd
  3. Mar 20, 2012 #2

    Borek

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    Staff: Mentor

    No, 0.01g is wrong.

    In general your approach looks OK to me, with one exception. Where did you get the 1.8 for a "dissociation constant" from?
     
  4. Mar 20, 2012 #3

    Wek

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    The 1.8 dissociation constant was given.

    How much would the the NaCl grams of a 0.1% solution be then?
    The way I calculated the NaCl grams was by reasoning that if 1% = 1g/100ml then in 1000ml there would be 10g (10g/1000ml = 1%). So for a 1 liter of 0.1% NaCl solution there would be 10g*(0.1/100) = 0.01g.
     
    Last edited: Mar 20, 2012
  5. Mar 20, 2012 #4

    Borek

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    Staff: Mentor

    Correct, but

    this is wrong and I can't even understand what you did.

    Try to directly apply percentage definition:

    [tex]\frac{\text{mass of NaCl}}{\text{mass of 1L of solution}} 100\% = 0.1\%[/tex]
     
  6. Mar 20, 2012 #5

    Wek

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    Hm. If I use that formula I would need to use 0.001g of NaCl per 1kg of water to get a 0.1% solution.

    (0.001g/1kg)*(100) = 0.1%

    So to make 1 liter of 0.1% NaCl I need 0.001g of NaCl?
     
  7. Mar 21, 2012 #6

    Borek

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    Staff: Mentor

    You divide grams per kilograms and you think it is OK?

    Using your approach if you have 1 cent and I have 1 dollar we have both the same amount of money, as 1=1. You have to watch your units, always.
     
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