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PH question

  1. May 19, 2009 #1
    1. The problem statement, all variables and given/known data

    hey guys, ive got a prob, i kinda know how to do this question but i cant get past the first step. i want to write an equilbrium expression by im not sure what the chemical reaction is. any help would be good

    2. Relevant equations

    A solution of a weak acid, HCOOH, is made to a concentration of 0.689 mol / L. Calculate the pH of this solution.
    Ka of HCOOH is 1.8 x 10-4.


    3. The attempt at a solution
     
  2. jcsd
  3. May 19, 2009 #2

    symbolipoint

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    The reaction of acetic acid in water (after actually dissolving) is a neutralization reaction. The water acts as a base. Also, for writing the Ka expression, the concentration of water does not effectively change. Since it is essentially constant, it is not written as part of the Ka expression.

    Your reported value for Ka is one magnitude wrong. It should be 1.8*10^(-5), which is ten times smaller than the value you showed.
     
    Last edited: May 19, 2009
  4. May 19, 2009 #3

    Borek

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    Actually question is about formic acid and Ka value of 1.8x10-4 is correct.

    Check calculation of pH of a weak acid lecture.
     
  5. May 19, 2009 #4

    symbolipoint

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