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Which of the following equations correspond to the Ka2 for phosphoric acid?
note: the double equal sign (==) means equilibrium.

a) HPO42- (aq) + H2O (l) == H3O+(aq) + PO43-(aq)

b) PO43- (aq) + H2O (l) == HPO42-(aq) + OH–(aq)

c) H3PO4 (aq) + H2O (l) == H3O+(aq) + H2PO4-(aq)

d) H3PO4 (aq) + 2 H2O (l) == 2 H3O+(aq) + HPO42-(aq)

e) H2PO4- (aq) + H2O (l) == H3O+(aq) + HPO42-(aq)

My answer is PO4^3-^ + H2O == HPO4^2-^ + OH^-^. Would you agree?

Here are the three ionizations for H3PO4. If you write these you will know the answer.
H3PO4 + H2O ==> H3O^+ + H2PO4^-
Now you write 2 and 3.

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