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.

No, your answer is not correct. The equation that corresponds to Ka2 for phosphoric acid is:

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

In this equation, phosphoric acid (H3PO4) reacts with two water molecules (H2O) to form two hydronium ions (H3O+) and one hydrogen phosphate ion (HPO42-). This equation represents the second dissociation of phosphoric acid.

To determine which equation corresponds to the Ka2 for phosphoric acid, we need to understand the definition of Ka2 and the dissociation of phosphoric acid.

Ka2 is the acid dissociation constant for the second ionization step of phosphoric acid, specifically for the formation of HPO42- from H2PO4-. It represents the equilibrium constant for the reaction:

H2PO4- (aq) ⇌ HPO42- (aq) + H+ (aq)

Looking at the given equations:

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

This equation represents the equilibrium constant for the first ionization step of phosphoric acid, not the second ionization step (Ka2). So, equation (a) is not the correct answer.

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

This equation represents the equilibrium constant for the reaction of phosphate ion (PO43-) with water, resulting in the formation of the hydrogen phosphate ion (HPO42-) and hydroxide ion (OH-). It does not directly correspond to the second ionization step of phosphoric acid. Therefore, equation (b) is not the correct answer.

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

This equation represents the equilibrium constant for the dissociation of phosphoric acid into hydronium ion (H3O+) and dihydrogen phosphate ion (H2PO4-). It corresponds to the first ionization step of phosphoric acid, not the second ionization step. Thus, equation (c) is not the correct answer.

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

This equation represents the equilibrium constant for the second ionization step of phosphoric acid, where H3PO4 donates two protons (H+) to water, resulting in the formation of hydronium ions (H3O+) and hydrogen phosphate ion (HPO42-). Therefore, equation (d) is the correct answer for Ka2.

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

This equation represents the equilibrium constant for the reverse reaction of the second ionization step of phosphoric acid, not the second ionization step itself. Hence, equation (e) is not the correct answer.

Therefore, the correct equation corresponding to Ka2 for phosphoric acid is:

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

Your answer, PO4^3- + H2O == HPO4^2- + OH^-, does not match the correct equation for Ka2.