The general acid dissociation constant (K_a) expression for an acid HA reacting with water is:

Alright ~Mataa🫶🏽, let's write these $K_a$ expressions.

The general acid dissociation constant ($K_a$) expression for an acid $HA$ reacting with water is:
$HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$
$K_a = \frac{[H_3O^+][A^-]}{[HA]}$

• a) $H_2PO_4^-$ (Dihydrogen phosphate ion)

  $$H_2PO_4^-(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + HPO_4^{2-}(aq)$$ $$K_a = \boxed{\frac{[H_3O^+][HPO_4^{2-}]}{[H_2PO_4^-]}}$$

• b) $H_3PO_2$ (Hypophosphorous acid)

  $$H_3PO_2(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + H_2PO_2^-(aq)$$ $$K_a = \boxed{\frac{[H_3O^+][H_2PO_2^-]}{[H_3PO_2]}}$$

• c) $HSO_4^-$ (Hydrogen sulfate ion)

  $$HSO_4^-(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + SO_4^{2-}(aq)$$ $$K_a = \boxed{\frac{[H_3O^+][SO_4^{2-}]}{[HSO_4^-]}}$$