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

Mole fraction concept help

  1. Sep 29, 2011 #1
    It looks like my first post on this did not make it on this forum some how.

    I came across this statement.
    Even though [tex]\frac{N_i}{N}=X_i[/tex], [tex]\frac{dN_i}{N}\neq dX_i[/tex]

    How does this work? The book offered no help as well as searches on the internet.
  2. jcsd
  3. Sep 29, 2011 #2


    User Avatar

    Staff: Mentor

    My guess is that as N is sum of all Nk, if Ni changes, denominator changes as well.
  4. Sep 29, 2011 #3
    I guess that makes sense. Seems logical to me.
  5. Sep 29, 2011 #4
    Yep, that's it.

    [itex]dx_i = \frac{{N \cdot dN_i - N_i \cdot dN}}{{N^2 }}[/itex]


    [itex]dN = dN_i[/itex]


    [itex]dx_i = \frac{{N - N_i }}{{N^2 }} \cdot dN_i[/itex]

    But for small [itex]x_i[/itex]

    [itex]dx_i = \frac{{dN_i }}{N}[/itex]

    is a good approximation.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook