Pka of carboxylic acid in aspirin?

What is the pKa of aspirin’s carboxylic acid?

Aspirin (acetylsalicylic acid) has a carboxylic acid group (–COOH). The pKa of that carboxyl group is about 3.5.

Why that pKa matters (what it means in practice)

A pKa around 3.5 means aspirin is mostly in its deprotonated (–COO⁻) form when the surrounding pH is above ~3.5, and mostly in its protonated (–COOH) form when the pH is below ~3.5. This influences solubility and how much of the drug is absorbed in different parts of the body.

Does aspirin have other ionizable groups that could change the answer?

Aspirin’s pKa value people usually quote refers to the carboxylic acid group. Aspirin also contains an aromatic ring with a phenyl group, but the key ionizable group relevant to acid-base behavior is the carboxylic acid.