Keq Concept: No Liquids in Keq

[zmike]

So I see in my book that for an acid dissociation, the Ka = K eq [H 2 O]
and that the Keq includes H2O in the eqn. Shouldn't there be no liquids in Keq?

Keq = [H 3 O + ] [A - ]
[HA] [H 2 O]

Why is there H2O in Keq?
If I am asked to calculate Keq do I have I have to include [H2O]?

Thanks
_______________

 

[DrDu]

That's a question of standard state. For diluted substances you use, simplified, the concentration c divided by the concentration [itex] c^0[/itex] =1 mole/l, e.g. [itex] [HA]=c_{HA}/c^0_{HA} [/itex]. For the solvent you use as standard state the pure solvent that is for water [itex] c^0=[/itex] 55.5 mole/l.
So the concentration of H2O in a dilute solution of the acid is very nearly the concentration of H2O in pure water, hence the quotient which enters the equilibrium factor is nearly 1.