"what is the expression for kw, the ion–product constant for water"

The ionization reaction of water is as follows:

The expression for ionization constant is written as follows:

In the expression, reactant and product only in gaseous and aqueous form are written. Reactant and product in solid and liquid state are not written in the expression for ionization constant. Thus, will not be there in the expression.

Therefore, expression for ionization constant of water will be:

Here, is ionization constant of water, is concentration of hydrogen ion and is concentration of hydroxide ion.

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every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.