Acid dissociation constant or acidity constant (symbol: Ka) of the hypothetical compound HA is the equilibrium constant of the reaction (see,Brønsted-Lowry theory),
in dilute aqueous solution at 25 ºC.
For the above system,
In dilute solution,
The larger the Ka of a species, the larger the fraction of the species that donates H+ to water, greater its acid strength. Thus, Ka of a species is a measure of its acid strength.
Ka of HI is larger than that of HCl, meaning HI is a stronger acid than HCl.