The acid dissociation constant (Ka) is calculated using which formula?

The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It reflects how completely an acid dissociates into its constituent ions in an aqueous solution. When an acid (HA) dissociates, it breaks down into hydrogen ions ([H+]) and its conjugate base ([A-]). The formula for the acid dissociation constant is derived from the equilibrium concentrations of these species at equilibrium.

The correct formula represents this relationship as:

[ Ka = \frac{[H^+][A^-]}{[HA]} ]

This shows that Ka is proportional to the product of the concentrations of the hydrogen ions and the conjugate base, divided by the concentration of the undissociated acid. The concentration of the undissociated acid ([HA]) appears in the denominator because as the dissociation of the acid increases, the concentration of [HA] decreases. Therefore, a larger Ka indicates a stronger acid that dissociates more completely.

In this case, option C accurately captures this relationship, establishing Ka as a reflection of the balance between these species in solution.