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Acid base confusion

  1. Nov 16, 2015 #1
    • Please pay attention to the form of your posts, such short lines make it extremally difficult to read.
    Hey all!!
    I have a silly doubt
    In acids and bases
    As you guys know
    The concentration of H+ ions
    For a weak acid
    Which does not undergo
    Complete dissociation
    Is given by mod H+=square root of Ka times C
    Where C is the initial concentration
    Of the acid
    And k is the ionization constant
    In the same manner
    For a weak base
    The concentration of OH- ions
    Is given in a similar manner
    And ka is replaced by kb (assuming C remains constant for both of them)
    Now assume that the acid and the base both are at 25 degrees centigrade
    Thus kw=10^-14
    Now multiplying mod H+
    And OH-
    We get 10^-14=10^-7 (square root of kw) times C
    And thus C comes out to be 10^-7
    Here is where the contradiction arises
    If the concentration of the acid is less than 10^-7
    And same for the base
    Dissociation does occur
    And their respective ph
    Can be found
    So why does C appear to be only 10^-7
    Why a constant
    Because concentration varies as per our will
    We can add more solute in a solution or decrease the amount
    If we want to!!
    Help is much appreciated
  2. jcsd
  3. Nov 16, 2015 #2
    My doubt
    Got cleared
    I wasn't thinking properly
    Thanks anyways
  4. Nov 16, 2015 #3


    User Avatar

    Staff: Mentor

    This is only an approximation, which holds under specific circumstances - you can't use it outside of the applicability range.

    See http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base for a discussion.
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