For acid dissociation in water we can write the general equation:
HA(aq)+H2O(aq)⇌H3O++A−
For strong acids, i.e. HNO3,HX,H2SO4, the equilibrium lies strongly to the right, and we can normally treat these solutions as stoichiometric in H3O+.
But for weaker acids, e.g. HNO2,H3C−CO2H, at equilibrium, some of the acid will remain undissociated and we use Ka to quantify the equilibrium:
i.e. Ka = [H3O+][A−][HA]
And thus for weak acids, Ka is small, whereas for stronger acids, such as those listed above, Ka is large, and the acid may undergo almost complete dissociation.
