What is Ka?

##K_a## is the acid dissociation constant that quantifies the behaviour of an acid in aqueous solution.

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 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.