Organic Chemistry MCAT Practice Exam 2025 - Free MCAT Practice Questions and Study Guide

The acidity of a carboxylic acid is primarily determined by the stability of the carboxylate ion that forms when the acid donates a proton (H+). When a carboxylic acid ionizes, it loses a proton to yield a negatively charged carboxylate ion. The stability of this ion is crucial; the more stable the carboxylate ion, the stronger the acid.

Several factors can enhance this stability, such as resonance stabilization. In the case of carboxylate ions, the negative charge can be delocalized over multiple electronegative oxygen atoms, which significantly stabilizes the ion. Factors like electronegativity of substituents on the carboxylic acid can also influence stability; electronegative atoms or groups attached to the carbon chain can help stabilize the negative charge through inductive effects or additional resonance structures.

The other options, while they pertain to characteristics of molecules, do not directly correlate with acidity as effectively as the stability of the carboxylate ion. Molecular weight, for instance, does not affect acid strength; it only influences physical properties. Electronegativity and electron-withdrawing or donating groups can impact the ion's stability, but the overall measurement of