For which of the following equilibria does kc correspond to the acid-dissociation constant, ka, of h2po4-?

a) h2po4-(aq) + h3o+(> h3po4(aq) + h2o(l)
b) h2po4-(aq) + h2o(> h3po4(aq) + oh-(aq)
c)h2po4-(aq) + h2o(> h3o+(aq) + hpo42(aq)
d) h3po4(aq) + h2o(> h3o+(aq) + h2po4-(aq)
e) hpo42-(aq) + h2o(> h2po4-(aq) + oh-(aq)

C) H₂PO₄⁻(aq) + H₂O(l) → H₃O⁺(aq) + HPO₄²⁻(aq)

Explanation:

For which of the following equilibria does Kc correspond to the acid-dissociation constant, Ka, of H₂PO₄⁻?

A) H₂PO₄⁻(aq) + H₃O⁺(aq) → H₃PO₄(aq) + H₂O(l)

NO. This is the inverse of the acid dissociation of H₃PO₄.

B) H₂PO₄⁻(aq) + H₂O(l) → H₃PO₄(aq) + OH⁻(aq)

NO. This is the basic dissociation of H₂PO₄⁻.

C) H₂PO₄⁻(aq) + H₂O(l) → H₃O⁺(aq) + HPO₄²⁻(aq)

YES. This is the acid dissociation of H₂PO₄⁻. The acid-dissociation constant is:

D) H₃PO₄(aq) + H₂O(l) → H₃O⁺(aq) + H₂PO₄⁻(aq)

No. This is the acid dissociation of H₃PO₄.

E) HPO₄²⁻(aq) + H₂O(l) → H₂PO₄⁻(aq) + OH⁻(aq)

NO. This is the basic dissociation of HPO₄²⁻.

the answer is 75.0%.