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What is the concentration of acetic acid in 250mL of a 100mM acetate buffer at pH4.76

  1. Feb 2, 2013 #1

    EKG

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    1. The problem statement, all variables and given/known data
    What is the concentration of acetic acid in 250mL of a 100mM acetate buffer at pH 4.76?


    2. Relevant equations
    Henderson Hasselback


    3. The attempt at a solution
    I used the Henderson Hasselback equation. plugged in 4.76 for pKa. plugged in .025 for the concentration of A-. plugged in 4.76 for pH. Solved and got 25mM. The answer is supposed to be 50mM. Would appreciate the help, thanks.
     
  2. jcsd
  3. Feb 3, 2013 #2

    epenguin

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    Homework Helper
    Gold Member

    Re: What is the concentration of acetic acid in 250mL of a 100mM acetate buffer at pH

    You don't give any reason for having plugged in 0.25 for the molarity of A-.

    Anyway time to fix in mind the often useful fact that when pH = pK then [A-] = [HA] and get clear why. Since here pH does = pK, when you plug in any value for [A-] you will get the same value for [HA], as you did.
     
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