Equlibrium Constant Expression for Strong Acids/Bases

[Lindsey Chheng 1E]

Why do Ka and Kb not exist for strong acids/bases?

[Ruby Tang 2J]

Ka and Kb are meant to be used to calculate equilibrium concentrations of weak acids and bases only, because generally only a very tiny fraction of the acid/base is dissociated in water if it is weak. Strong acids and bases will have very large Ka and Kb, because they are generally assumed to completely dissociate, so you don't need to use their Ka/Kb to calculate percent dissociation of the acid/base in water. You would just assume that the reaction goes to completion (ie 100% dissociation).

[Charisse Vu 1H]

Ka and Kb don't exist for strong acids and bases because they dissociate completely in solution. If you recall the formula for calculating the equilibrium constants,
you will see that the concentration of the acid itself is in the denominator. Since strong acids/bases dissociate completely, the concentration of the acid/base would be close to zero and the equilibrium constant would be irrelevant. Weak acids and bases don't dissociate completely, so an equilibrium constant must be calculated.

[rohun2H]

Strong acids/bases are assumed, when approximating, to reach 100% ionization. Therefore, the denominator of the equilibrium constant equation is essentially zero which is why we do not use the equilibrium constant for strong acids/bases.