Final answer:
The compound that is a strong acid among the given options is a. HNO₃ (nitric acid), as it completely dissociates in water, unlike the other options which include a weak base, a salt, and a diatomic molecule.
Step-by-step explanation:
The answer to the question, “Which one of the following is a strong acid?” is a. HNO₃ (nitric acid). NH₃ (ammonia) is a weak base, CaSO₄ (calcium sulfate) is a salt, and H₂ (hydrogen gas) is a diatomic molecule, not an acid. Nitric acid is considered a strong acid because it dissociates completely in water, producing a high concentration of hydrogen ions (H+).
Strong acids, such as HNO₃, are characterized by their complete ionization in aqueous solution. This is in contrast to weak acids, which only partially ionize. An example of a weak acid is CH₃CO₂H (acetic acid), and for bases, compounds like NH₃ which partially accept protons in solution, are considered weak bases.
Understanding the concept of acid strength is important when predicting the behavior of acids and bases in chemical reactions. Acid-base equilibria are governed by the degree to which an acid or base ionizes, with strong acids and bases ionizing completely whereas weak acids and bases only do so partially.