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.
