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March 27, 2017

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Calculate the theoretical pH at the starting point and at each equivalence point for phosphoric acid, acetic acid, and sulfuric acid. Please show the equilibrium equation, the Ka, and the pH calculation.

  • Chemistry - ,

    I could write a book on this but I don't have the time. Here is how you do acetic acid. The others are done the same way.
    CH3COOH + NaOH(for example) ==> CH3COONa + H2O
    So the pH at the equivalence point is determined by the hydrolysis of the salt.
    CH3COO^- + HOH ==> CH3COOH + OH^-
    Kb = (Kw/Ka) = (CH3COOH)(OH^-)/(CH3COO^-)
    This is as far as you can go on ANY of the above until you have a concentration of each acid.
    After you have that, Kw = 1E-14
    Ka = Ka for CH3COOH from your text or notes.
    (CH3COOH) = x
    (OH^-) = x
    (CH3COO^-) = Molarity -x
    Solve for x (which is OH^-), then convert to pOH using pOH = -log(OH^-) and use
    pH + pOH = pKw = 14 and solve for pH.
    Post your work if you get stuck.

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