This is somewhat difficult to understand. In the gaseous state it is included. In reactions where it is a product it is included;e.g. alcohol + acid --> ester + H2O. In aqueous solution, it is not included for it is essentially a constant. In
NaOH(aq) + HCl(aq) ==> NaCl(aq) + H2O(aq)
In this acid/base neutralization the water is essentially 1000g/18 = about 55.5 M anywhere in the titration and the Keq numerica constant really is 55.5 times a K which ends up being another constant which we call Kc in aqueous solution. I encourage any other tutor to post his/her thoughts
Another way to look at this is that H2O is not included in aqueous solution for the same reason that we do not include solids; i.e., in solutions we consider the water s a pure substance. Thermodynamically, solids, by definition, have an activity of 1. Same for "pure" liquids.
I know you're not supposed to consider water for equilibrium calculations (ICE tables and Kc) if it is liquid, but what if it is aqueous or gaseous? Thanks!
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