The Ka of HCl is 1.3 x 10^6. This is constituted as a large Ka. Hydrochloric acid is a strong acid and has complete dissociation in aqueous solution. Therefore, its Ka approaches infinity.
The acid dissociation constant, or Ka, is a way of ranking the strength of acids. Acids with lower Ka values are called weak acids. These acids dissociate only partially. Examples of weak acids are acetic acid (vinegar), ascorbic acid (vitamin C) and formic acid.
Polyprotic acids can lose more than one proton. These acids have Ka values for the loss of each proton. In general, the Ka reduces as each successive proton is lost.