ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

The correct answer is phosphoric acid (H3PO4), as it is not classified as one of the seven strong acids. Strong acids are characterized by their complete dissociation in water, which means they fully ionize to produce hydrogen ions (H⁺) and their corresponding anions. The common strong acids include hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4), all of which display this behavior.

In contrast, phosphoric acid is considered a weak acid because it does not fully dissociate in solution. When H3PO4 is added to water, it only partially ionizes, which results in an equilibrium that contains both the undissociated acid and its ions. This behavior leads to a significantly lower concentration of hydrogen ions compared to the strong acids, which provide a higher concentration of ions and therefore a more significant acid effect in solution.

Understanding these distinctions helps clarify why phosphoric acid is not listed among the strong acids, and knowing the characteristics of strong acids can aid in predicting their behavior in various chemical reactions and during acid-base titrations.