Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Molar mass

  1. Apr 13, 2005 #1
    To determine the molar mass of a certain protein, 1.00E-3 grams of the protein was dissolved in enough water to make a 1.00 mL solution. The osmotic pressure of this solution was found to be 1.12 torr at 25.0C. Calculate the molar mass of the protein.
  2. jcsd
  3. Apr 13, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes ? And what do you want us to do ?
  4. Apr 13, 2005 #3
    I assume you want help in working out the molar mass. Where are your attempts first???

    The Bob (2004 ©)
  5. Apr 13, 2005 #4
    I tried using this equation: p = i M R T, but I have no idea what i is.
  6. Apr 13, 2005 #5


    User Avatar
    Science Advisor

    i = moles of stand-alone stuff per mole of stuff (for lack of better words). for example, i for sulphuric acid is 3. i for potassium chloride is 2. i for acetic acid is 1 (weak acids do not dissociate).
  7. Apr 13, 2005 #6
    Do you know what it is for protein?
  8. Apr 13, 2005 #7


    User Avatar
    Science Advisor

    Probably 1
  9. Apr 13, 2005 #8
    i = van't Hoff factor. Roughly speaking, it measures the degree to which the ions dissassociate in solution. For example, the van't Hoff factor of NaCl is 2 since NaCl completely dissassociatesin water. Since most proteins don't ionize in water, n is probably 1.

    i does not necessarily have to be an integer. For compounds that don't completely dissassociate, the van't Hoff factor is less than wht you would expect. The van't Hoff factor for HF for instance, is only a little bit more than 1 and not 2 since HF is a weak acid.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook