Which of the following is a weak acid in aqueous solution? — Acids and Bases Chemistry Question

STEPS:

1. Define Acid Strength: The strength of an acid in aqueous solution is determined by its extent of ionization (or dissociation). According to the sources, strong acids are those that dissociate completely into ions, whereas weak acids only partially ionize, resulting in an equilibrium between the molecular acid and its ions.
2. Identify the Core Concept: The question tests the ability to distinguish between the small group of common strong acids and the much larger category of weak acids.
3. Evaluate the "Strong Acid" List: In AP Chemistry, students are expected to recognize the standard strong acids:
* Binary Acids: $HCl$, $HBr$, and $HI$.
* Oxyacids: $HNO_3$, $H_2SO_4$, $HClO_4$, and $HClO_3$.
4. Analyze the Options:
* Options A, B, C, and E ($HCl$, $HClO_4$, $HNO_3$, and $H_2SO_4$) are all included in the standard list of strong acids.
* Option D ($H_2S$) is a binary acid, but it is not one of the three strong halogen-based binary acids.
5. Conclusion: Because $H_2S$ (hydrosulfuric acid) does not dissociate completely in water and is not one of the recognized strong acids, it is the only weak acid among the choices provided.

WHY_OTHERS_WRONG:

• $HCl$ (A), $HClO_4$ (B), and $HNO_3$ (C): These are all monoprotic strong acids. In aqueous solution, they ionize 100%, meaning there are no intact acid molecules left in the solution, which is the opposite of how a weak acid behaves.
• $H_2SO_4$ (E): Sulfuric acid is a diprotic strong acid. While its second dissociation step is weak, the first proton dissociates completely in water ($H_2SO_4 \rightarrow H^+ + HSO_4^-$), leading to its classification as a strong acid.