Final answer:
The correct expression for the acid dissociation constant (Ka) is d) H3O+A-/HA, representing the ratio of the concentrations of the hydronium ion and the conjugate base to the concentration of the weak acid at equilibrium.
Step-by-step explanation:
The correct expression for the acid dissociation constant (Ka) is d) H3O+A-/HA. The acid dissociation constant is the equilibrium constant for the ionization of a weak acid. In the dissociation reaction, a weak acid (HA) reacts with water to form the hydronium ion (H3O+) and the acid's conjugate base (A-). The Ka is given by the ratio of the concentrations of the products to the concentration of the reactants, with the concentration of water assumed to be a constant. Therefore, Ka for the ionization of HA is represented as [H3O+][A-] / [HA]. This is true when the reversible reaction reaches its equilibrium state.