[positiveion]

[Borek]

Concentration of water is assumed to be constant, so it is moved into Keq value.

[sjb]

BF₃ (aq) + 3 H₂O (aq)  3 HF(aq) + H₃BO₃ (aq)

Keq = [HF]³ x [H₃BO₃] / [BF₃]

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The main reason that water is not considered in this instance is because it's the solvent, any changes in its concentration as a result of the reaction will be very small, effectively a constant in this case. If the reaction were much more concentrated then it would probably be best to include it as well.

[dkasprick]

Isnt it because pure solids and pure liquids arent included in equilibrium  constant expressions ?

[Borek]

No, that holds when they are in separate phases.