For acid dissociation in water we can write the general equation:
#HA(aq) + H_2O(aq)rightleftharpoonsH_3O^+ + A^-#
For strong acids, i.e. #HNO_3, HX, H_2SO_4#, the equilibrium lies strongly to the right, and we can normally treat these solutions as stoichiometric in #H_3O^+#.
But for weaker acids, e.g. #HNO_2, H_3C-CO_2H#, at equilibrium, some of the acid will remain undissociated and we use #K_a# to quantify the equilibrium:
i.e. #K_a# #=# #([H_3O^+][A^-])/([HA])#
And thus for weak acids, #K_a# is small, whereas for stronger acids, such as those listed above, #K_a# is large, and the acid may undergo almost complete dissociation.
