HX ⇌ H+ + X-
is defined as the ratio of the concentrations of the products (H+ and X-) to the concentration of the reactants (HX) at equilibrium:
$$Ka = \frac{[H^+][X^-]}{[HX]}$$
The square brackets [ ] denote the concentrations of the respective species.
The Ka value quantifies the acidity of HX. A higher Ka value indicates a stronger acid, meaning it dissociates more completely into H+ and X- ions, whereas a lower Ka value indicates a weaker acid.
