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I read somewhere that hydrogen cation combines with water to form hydronium ion. I also read it combines with various water molecules. Can someone please explain?

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    $\begingroup$ Have you tried searching and reading e.g. about hydronium ? $\endgroup$
    – Poutnik
    Oct 10, 2020 at 14:01
  • $\begingroup$ If a question is asked on Chemistry SE site, then, in contrary to sites like Quora, it is expected the author elaborates the question by at least basic textbook/online search and writes down what he/she found/understood and what is the eventual stumble stone. The "quick questions" without explicitly expressed particular effort are not very welcome, and may be closed. OTOH, explicitly written effort significantly raises probability of receiving appreciated answers. $\endgroup$
    – Poutnik
    Oct 10, 2020 at 14:02

1 Answer 1

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Usually a pure acidic molecule like HCl gives one proton $\ce{H+}$ to a base like NH3 or $\ce{CH3COO-}$, and the proton once fixed on the base gives a new species which is $\ce{NH4^+}$ or $\ce{CH3COOH}$. And that's all what's happening. With water, it is a bit different. The $\ce{HCl}$ molecule starts by giving one proton $\ce{H+}$ to one $\ce{H2O}$. This produces an ion $\ce{H3O+}$ and usually chemists consider that it is the final result. It is not, because it is possible to show that the proton may attract a second water molecule in order to make a heavier ion $\ce{H5O2^+}$ having a charged H atom in a central bridge $\ce{H2O-H^+-OH2}$. Thermodynamic measurements are even able to show that still more $\ce{H2O}$ molecules are attracted around this $\ce{H5O2^+}$ ion, to produce an ion $\ce{H_{2n+1}O_n^+}$. The trouble is that the exact value of the parameter $n$ is not known with precision. It should be between $6$ and $10$. That is why many chemists forget about this difficulty and have decided that the only species in water is $\ce{H3O^+}$. This simplifies also the determination of dissociation constants of acids in water.

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