Butanoic acid, with the molecular formula HC4H7O2, is classified as a weak acid. When dissolved in water, it undergoes a dissociation reaction where it donates a proton (H+) to water. This process can be illustrated by the following chemical equation:
HC4H7O2 (aq) + H2O (l) <=> H3O+ (aq) + C4H7O2- (aq)
In this equation, butanoic acid reacts with water to produce hydronium ions (H3O+) and butanoate ions (C4H7O2–).
The acid dissociation constant, Ka, of butanoic acid is 1.5. This constant is a measure of the strength of the acid in solution and indicates the extent to which the acid can dissociate into its ions:
Ka = [H3O+][C4H7O2-] / [HC4H7O2]
A larger Ka value suggests a stronger acid, with more dissociation into ions, while a smaller value indicates a weaker acid with less dissociation. Since butanoic acid has a Ka of 1.5, it demonstrates that it does not completely dissociate in water, characteristic of weak acids.