What is Ka?

1 Answer
Oct 12, 2016

Answer:

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

Explanation:

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.