Weak Acids and Bases and Ksp

[homestar]

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From what I understand, pure liquids and solids do not appear in an equilibrium constant expression: Ksp. This is why water does not appear in a Kw expression.

Yet, when I see Ka and Kb expressions, the weak acid or base is included in the denominator of the expression. I am confused by this. Are they not pure? Can anyone shed light on this?

Best,
Homestar

 

[RogueUnicorn]

They are solvated in water, and thus are aqueous, not in a pure liquid or solid state.