Which equation correctly describes the equilibrium constant for acids (Ka) in the context of solvent leveling?

To determine the correct equation that describes the equilibrium constant for acids (Ka) in the context of solvent leveling, it's essential to understand what Ka represents. The acid dissociation constant (Ka) quantifies the extent to which an acid dissociates into its constituent ions in a solution.

In this context, we are considering a generic weak acid (HA) dissolving in a solvent. When HA donates a proton (H+), it forms its conjugate base (A-) and a hydronium ion (H3O+), which can be simplified in many contexts to [H2O], particularly when considering the solvent as highly stabilizing.

The equilibrium reaction for the dissociation of the acid can be expressed as:

HA ⇌ H3O+ + A-

The equilibrium expression for this reaction would typically be:

Ka = [H3O+][A-] / [HA]

However, in the context of solvent leveling, the concentration of water (assuming it is a pure solvent) remains effectively constant due to its large excess compared to the acid and its ions. Therefore, it is often included in the equilibrium constant as a constant term, effectively altering the expression to focus solely on the concentrations of solute species.

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