Question

What is the ion-product constant for water (kw) give the definition the expression and the value

Answer 1

The ion product if H2O is the product of [H3O+] (or [H+] depending on your book) and [OH-] at a specific temperature, and this product is derived from the equilibrium constant for the reaction of self-ionization of H2O. The expression is as follows
Kw = [H3O+][OH-]
(Or Kw = [H+][OH-] depending on your book)

And the value of Kw is 10^-14 at 25 °C

Hope this helps