In this reaction H2O serves as the base. The hydronium ion, H3O+, is the conjugate
acid of H2O, and the chloride ion is the conjugate base of HCl. It is the hydronium
ion that is the acidic species in solution, and its concentration determines
the acidity of the resulting solution. We have chosen to use a single arrow
(®) in place of the double arrows (t) to indicate that we treat HCl as if it were
completely dissociated in aqueous solutions. A solution of 0.10 M HCl is effectively
0.10 M in H3O+ and 0.10 M in Cl–. In aqueous solutions, the common
strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid
(HBr), nitric acid (HNO3), perchloric acid (HClO4), and the first proton of sulfuric
acid (H2SO4).