EK 1001Qs #770 & #771 770. Hydrofluoric acid is a weak acid (pKa = 3.1). In terms of its ionization in water, fluoride is: A. a weak acid B. a weak base C. a strong acid D. a strong base The answer is B. Their explanation is that the pKb of fluoride is 11. So what pKb constitutes a weak base exactly ? I know the conjugate base of a weak acid can be either weak or strong, but how do you tell the difference? 771. What is the acid dissociation constant Ka of water? A. 1.0 x 10^-14 B. 1.8 x 10^-16 C. 1 D. 10 The answer is B. Their explanation is that Ka = (Kw)/55M, but where did this equation comes from? What's the difference between Ka and Kw for water? I don't really get it. If Kw is just based on autoionization of water, what is Ka?