[AntiStalker69]

When dealing with K_w (ionic product constant of water), why do we neglect the liquid (H2O). I understand that the probability of water dissociating is very little so the concentration of water changes very little. But why do we assume [H2O] as a constant, if the concentration of water changes very little.

Please explain to me like I am 5.

[Borek]

Hint: calculate change in the water concentration after dissociation. Is it measurable? Does inserting it into the Kw change calculation results?

Note that we know Kw with at most four significant digits. Compare accuracy lost by ignoring water concentration change with that of Kw being known with limited accuracy.

Besides, it is not like we don't use water in the Kw - yes, in typical applications we simply ignore it, but you will see Kw listed as at least two different values: https://www.chembuddy.com/?left=pH-calculation&right=water-ion-product