1. Dismiss Notice
  2. Guest, be sure to check out How To Get Into Dental School, our free downloadable PDF with step-by-step details for dental school applicants!
    Dismiss Notice
  3. Dismiss Notice
  4. Download free Tapatalk for iPhone or Tapatalk for Android for your phone and follow the SDN forums with push notifications.
    Dismiss Notice

mole fraction

Discussion in 'DAT Discussions' started by vvvv, May 28, 2008.

  1. vvvv

    2+ Year Member

    Joined:
    May 12, 2007
    Messages:
    119
    Likes Received:
    0
    Status:
    Pre-Dental
    gen chem destoyrer #136.they say mole fraction of a gas in water is 2x10^-18. then they set the mole of water is 1 mole, and calculate mole of gas is 2x10^-18. This means mole fraction = mole gas/mole water.
    But is that mole fraction = mole gas / (mole gas + mole water)???? help plz...thanks
     
  2. Note: SDN Members do not see this ad.

  3. dentalplan

    2+ Year Member

    Joined:
    Mar 1, 2008
    Messages:
    140
    Likes Received:
    0
    Status:
    Pre-Dental
    yes you are right, mole fraction is mole of gas / (mol of gas + mole of water).

    however, that question that you're talking about in destroyer......they tell you the mole fraction of the gas is 2 x 10^-18.

    A mole fraction is just that -- a fraction. But there is always another part(s) to a fraction. So......when you add all of the fractions of a whole up -- you are suppposed to get 1 (the whole).

    All you have to realize is that if you're talking about a 2 x 10^-18 gas mole fraction.
    The other part of the fraction (the h2o fraction) is going to be something very close to 1. Either way, when you add them, you will get 1.

    However......

    (1) - (2 x 10^-18) = some number reallyyyyyyyy close to 1. It comes out to something like .9999999999999999999999998 or something like that. Most calculators can't even figure that out. But it doesn't matter. Because when you add the gas mole fraction to the h2o mole fraction, that always equals to 1. And since the h2o mole fraction is so close to 1, you can round it to 1. This is assuming that we are looking at only 1 mole of h2o. So......you also know that 1 mole of h2o contains 18g, or....018 kg.

    And since molality = moles of gas / kg of h2o ........

    you know moles of gas when there is 1 mol of h2o, so...

    (2 x 10^-18) / (.018) = ~ 1 x 10^-16
     
  4. vvvv

    2+ Year Member

    Joined:
    May 12, 2007
    Messages:
    119
    Likes Received:
    0
    Status:
    Pre-Dental
    thanks it makes sense.
     

Share This Page