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

Homework Help: What is the molar mass of the unknown?

  1. Feb 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A 1.40 g sample of an unknown is dissolved in 8.10 g of lauric acid. The freezing point is depressed 1.23 oC. What is the molar mass of the unknown?



    2. Relevant equations

    Delta T = Kf * m



    3. The attempt at a solution

    Delta T = Kf * m
    => m = 1.23 / 3.9 = 0.32

    mole solute = 0.32 * 0.0081 = 0.0026 moles.

    => Molar mass = 1.40 / 0.0026 = 540 g/mol = final answer.

    Am I doing correct? Please let me know!
     
  2. jcsd
  3. Feb 16, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    Not bad. Two thngs that will make your solution better.

    1. Don't round down intermediate results (check that it changes molar mass by about 8g).
    2. Don't ignore units in your calculations, they are much easier to follow when it is obvious what is what of what.
     
    Last edited by a moderator: Aug 13, 2013
  4. Feb 16, 2010 #3
    Thanks so much for your response!

    I have edited my answer to:

    Delta T = Kf * m => m = 1.23 / 3.9 = 0.32 m

    mole solute = 0.32 * 0.0081 = 0.0026 mole

    Molar mass of the unknown = 1.40 / 0.0026 = 538.46 g/mol .

    Hope it better ;)
     
  5. Feb 16, 2010 #4

    Borek

    User Avatar

    Staff: Mentor

    This time you overdid - you don't have enough information to give 5 significant digits in the answer :smile: But you see that rounding errors were responsible for 538 being 530.
     
    Last edited by a moderator: Aug 13, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook