Equilibrium constant is calculated by taking active masses of pure liquids as unity. But while calculating ionic product of water we could have simply defined the equilibrium constant as water constant.

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    $\begingroup$ I'm a bit confused. Are you asking why $$K_w = \ce{[H^+][OH^-]}$$ instead of $$K_w = \dfrac{\ce{[H^+][OH^-]}}{\ce{[H2O]}}$$ $\endgroup$ – MaxW Jan 7 '19 at 10:32
  • $\begingroup$ If I understand your question correctly, reading about equilibrium constant of water on Wikipedia should answer your question. If there is something else down the rabbit hole that's not clear to you, then you probably should re-formulate the question or add some more details. $\endgroup$ – andselisk Jan 7 '19 at 12:13

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