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The pH of a Mixture of Weak Acids

  1. Feb 14, 2009 #1
    1. The problem statement, all variables and given/known data
    2.00M chlorus acid and 2.00M formic acid are mixed. What is the pH?


    2. Relevant equations
    Ka of chlorus acid is 1.1×10-2
    Ka of formic acid is 1.78×10-4


    3. The attempt at a solution
    For chlorus acid with quadratic equation to find x:
    x²+0.011x-0.022
    (-0.011[tex]\pm[/tex][tex]\sqrt{}0.000121-4(1)(-0.022)[/tex])/2
    (-0.011[tex]\pm[/tex]0.297)/2
    The positive number of the two choices is 0.143

    For formic acid, and the x being small enough to neglect, x is:
    x²/2=0.000178
    x²=0.000356
    x=0.0189

    When mixing weak acids, I think I was told that only the acid with the larger Ka value matters, meaning taking the negative log of chlorus acid's H ion gets -log0.143=0.845.

    I wasn't sure this was correct so then I tried adding the two H ion concentration from each acid to find their pH. I did 0.143+0.0189=0.162 and negative log that for 0.791.
    I'm not sure it is added this way, since I got a bit confused about the idea of adding molarity together, so I divided by 2 first then negative log for 1.09.

    Still I'm not sure this is correct so I think I took the wrong steps. How is this correctly done?
     
  2. jcsd
  3. Feb 14, 2009 #2

    Borek

    User Avatar

    Staff: Mentor

    It isn't entirely true - you can do it if the difference between both acids pKas is large enough and if the weaker acid is weak enough. That's the case here and 0.84 is a correct answer.

    Unfortunately, it is hard to be more specific.
     
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