The acid dissociation constant (Ka) for an acid dissolved in water is a measure of the strength of the acid. It is defined as the equilibrium constant for the dissociation of the acid into hydrogen ions (H+) and conjugate base (A-). The Ka value is calculated by dividing the concentration of hydrogen ions (H+) and conjugate base (A-) by the concentration of the undissociated acid (HA).

The equilibrium reaction for the dissociation of an acid HA in water can be represented as:

HA + H2O ⇌ H3O+ + A-

The acid dissociation constant (Ka) is expressed as:

Ka = [H3O+][A-]/[HA]

where [H3O+], [A-], and [HA] represent the equilibrium concentrations of hydrogen ions (H3O+), conjugate base (A-), and undissociated acid (HA), respectively.

A higher Ka value indicates a stronger acid, as it means that the acid dissociates more readily in water, resulting in a higher concentration of hydrogen ions (H+). Conversely, a lower Ka value indicates a weaker acid, as it dissociates less readily in water, resulting in a lower concentration of hydrogen ions (H+).