1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Why does [1H2O->1(H) + 1(OH)], yet [10^(-14)M=10^(-7)M+10^(-7)M]?

  1. Oct 7, 2012 #1
    Say I have 10(H2O) that ionizes into H and OH. The reaction would be

    10(H2O)→10(H) +10(OH)

    Because if you have 10 of something, and then then split it into 2 pieces, you have 10 of each piece.

    If I have 1 piece of paper and I tear it into 2 pieces, I have 1 piece in my right hand and 1 piece in my left hand. It makes sense.

    The reactions is
    1 full paper → 1 torn piece + 1 torn piece.

    So just like 1 torn piece of paper + 1 torn piece of paper ≠ 2 full pieces of paper,

    10(H)+10(OH) ≠ 20(H2O).

    But how the the heck is this following equation true:

    [itex]10^{-14}M(H2O) = 10^{-7}M(H^{+})+10^{-7}M(OH^{-})[/itex]

    Why is it not:

    [itex]10^{-7}M(H2O) = 10^{-7}M(H^{+})+10^{-7}M(OH^{-})[/itex]?
  2. jcsd
  3. Oct 7, 2012 #2
    I wonder if this question makes any sense or not?

    If I have 10 blue marbles and drop them into a Liter of water, the concentration will be 10 blue marbles/1L

    If I have 10 red marbles and drop them into a liter of water, the concentration will be 10 red marbles/1L

    With 10 blue marbles + 10 red marbles dropped into a Liter of water, I will obviously have 20 seperate marbles/1L

    But if each red and blue marble fused into a purple marble, I'd end up with 10 purple marbles/1L.

    What I'm saying is, the concentration of marbles/Liters stays the same. There is no reason to do any addition if the marbles are fusing into purple marbles.

    So why is there addition when speaking on this reaction:

    [itex]10^{-14}M(H2O) = 10^{-7}M(H^{+})+10^{-7}M(OH^{-})[/itex]

    How is concentration so different than quantity, that in one instance 1A+1B→1(AB), yet for another instance, 1M(A)+1M(B) → 2M(AB)
  4. Oct 8, 2012 #3


    User Avatar

    Staff: Mentor

    Please don't start separate threads when the problem is already discussed elsewhere.

    You started with an incorrect premise. No idea where you got it from. Correct equation is

    [tex]K_w = [H^+][OH^-] = 10^{-14}[/tex]


    Topic locked.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook