Should water (h2o) be included in the model as a reactant or product? if so, how should it be included?

The water should be omitted as a reactant or product to have a well-balanced equation only if the water reacts as a solvent.  

Explanation:

The water acts as a pure liquid and the increase of concentration of pure liquid is impossible. So, it is of no use when it acts as a solvent. As a product form water can be obtained as a liquid or gaseous substance. Their ability can increase the supply of hydrogen or oxygen to the reaction. When it is used as a main reactant it should be included.

As a role of solvent, it has no use in the chemical reaction. It has insignificant values as a role of solvent so it should be formatted while having the well-balanced equation. There are a lot of examples in which water is not included in the chemical reaction.

not sure if this what   you'r looking for would be percent composition

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