The hydronium ion is an important factor when dealing with chemical reactions that occur in aqueous solutions. Its concentration relative to hydroxide is a direct measure of the pH of a solution. It can be formed when an acid is present in water or simply in pure water. Its chemical formula is H3O+. It can also be formed by the combination of an H+ ion with a H2O molecule.
H2O(l)⇌OH−(aq)+H+(aq)
Here Water molecules act as a donor