A concentrated acid is an acid solution with a high molar concentration of hydrogen ions. For example, 16 mole hydrochloric acid has a higher concentration than a 0.5 mole solution of the same compound.
The concentration of hydrogen ions in any solution is measured by its pH or the power of hydrogen on a scale of 0 to 14. A concentrated solution of 1 mole hydrochloric acid has a pH of 0, while pure water is neutral and has a measure of 7 on the pH scale. A base such as sodium hydroxide has a pH of 14. A strong acid dissociates completely in water to form hydronium ions.