Water is a weak electrolyte so its ionisation is not complete and undissociated water is in equilibrium with its ions, H3O+ and OH. Since water gives H3O+ and OH both, it is called amphiprotic. Let us take pure water of density 1 g/cc. Its concentration would be 1000/18 = 55.56 molar. Do check out Chemistry Questions of other chapters too.
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