Which acid has the highest pKa value?

The pKa value is a measure of the acidity of a compound; it represents the equilibrium constant of an acid dissociation reaction. A higher pKa value indicates a weaker acid. In this context, acetic acid (CH₃COOH) is the correct answer because it is a weak acid compared to the others listed.

Hydrochloric acid (HCl) is a strong acid with a very low pKa, typically around -7, indicating it dissociates completely in solution. Sulfuric acid (H₂SO₄) is also a strong acid, particularly in its first dissociation, and has a low pKa as well. Nitric acid (HNO₃) is similarly a strong acid with a low pKa value.

In contrast, acetic acid, as a weak acid, has a significantly higher pKa value, approximately 4.76. This means that acetic acid does not dissociate completely in solution, exhibiting much weaker acidity than HCl, H₂SO₄, and HNO₃. Therefore, among the choices provided, acetic acid stands out as the acid with the highest pKa, affirming its status as the weakest acid in this comparison.