chemistry

Phosphoric acid is a triprotic acid with pKa's of 2.14, 6.86, and 12.4.The ionic form that predominates at pH 3.2 is?choose the corect answer
(a) H3PO4
(b) H2PO4^-
(c) HPO4^2-
(d) PO4^3-

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  1. I can't draw the titration curve for H3PO4 on this board but you can draw it on a piece of paper. You look at the curve and figure out what you have at certain points.
    Beginning is H3PO4 and pH is about 1.5 or so for a 1 M solution of H3PO4.
    First equivalence point is H2PO4^- and that occurs at pH = 1/2(pK1 + pK2) = about 4.5. So, between the beginning and the first equivalence the pH is somewhere between 1.5 or so and 4.5 and the species must be H3PO4 and H2PO4^-.
    For different values of pH, it is done the same way. The pH at the second equivalence point is 1/2(pK2 + pK3) and at the third is approximately sqrt (KwK3/salt) = about 13 for a 1 M concn of the salt.
    The equation for each equivalence point is
    H3PO4 + OH^- ==> H2O + H2PO4^-
    H2PO4^- + OH^- ==>H2O + HPO4^-2
    HPO4^-2 + OH^- ==>H2O + PO4^-3

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