Acid dissociation constant

[Regina90]

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Why is water included in the acid dissociation equation if its a pure liquid? I thought pure liquids and solids were excluded from k equilibrium equations, Ka is the same as Keq right? Except Ka applies specifically to acids? Please help!

 

[Hemorrage]

water isn't included. What is included is H3O+ which is different from H2O. Protons (H+) hardly ever exist in solution isolated, instead they typically get abstracted by any basic compound near them. Typically this is water because we use it as a solvent frequently and therefore it's in large supply.

 

[Shjanzey]

Why is water included in the acid dissociation equation if its a pure liquid? I thought pure liquids and solids were excluded from k equilibrium equations, Ka is the same as Keq right? Except Ka applies specifically to acids? Please help!

At any given time in water you have an equilibrium of H2O and H3O+ and by definition H3O+ is acidic. This is why.

The following is the reaction that takes place.

H2O + H2O --> H3O+ + OH-

pKa = -log(Ka)

Ka = ([H+][A-]) / [HA]