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: Freezing point depression error

  1. Nov 29, 2006 #1
    So we did an experiment of freezing point depression and my calculated molar mass values were larger than actual...
    First, the freezing point of the pure solvent had to be determined and so if some of the solvent was on the walls of the test tube opposed to in the solution how would this affect results?
    As far as i've gotten is that it would result in a greater actual molality than observed, but how would this then effect temperature change and molar mass?

    hope this is enough, please help
  2. jcsd
  3. Dec 1, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To help you, we must first guess what your experiment involved (measure FP of pure solvent; add known mass of solute; measure FP of solution; use known FP depression constant to calculate molality; and from masses of solute and solvent calculate molar mass?). It would have helped if you had included these details in the opening post.

    If the above guess is correct (please confirm whether they are), then, as you said, the real molality will be greater than the calculated molality, due to solvent on the walls. So, the calculation underestimates the molality.

    M'(calc) =M(real) - dM (where dM >0)

    How do you relate the molality to the molar mass? Write down the equation for that relationship. I imagine you used some fixed mass s of the solvent and dissolved some small mass m of the solute in it. If that's true, can you write down the equation relating M (the molality) to MM (the molar mass) using m and s?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook