Question

Write the stepwise acid-base reactions for the following ions in water. Write the correct acid or base
dissociation equilibrium constant (example: Ka2 or Kb1) for each step.
a)PO4^3- (write a reaction for all three steps)
b) H3N+ – CH2 – COOH (2 steps)
c)Using the stepwise reactions for A, what is the equilibrium constant for:
PO4^3- (aq) + 3 H2O (l)  H3 PO4
(aq) + 3 OH- (aq)

Answer 1

a)

PO43- + H2O ----> HPO4 2- + OH-

Kb1 = [HPO4 2-][OH-] / [PO4 3-]

HPO4 2- + H2O ----> H2PO4- + OH-

Kb2 = [H2PO4-] [OH-] / [HPO4 2-]

H2PO4- + H2O -----> H3PO4 + OH-

Kb3 = [H3PO4] [OH] / [H2PO4-]

b)

H3N+ - CH2 - COOH + H2O -----> H3N+ - CH2 - COO- + H3O+

Ka1 = [H3N+ - CH2 - COO-] [H3O+] / [H3N+ - CH2 - COOH]

H3N+ - CH2 - COO- + H2O -------> H2N - CH2 - COO- + H3O+

Ka2 = [H2N - CH2 - COO-] [ H3O+] / [H3N+ - CH2 - COO-]

c)

Adding the 3 reactions from a) we get

PO4 3- + 3H2O -----> H3PO4 + 3 OH-

K eq = Kb1 x Kb2 x Kb3