 Electron-donating substituents give weaker acids (the pKa of formic acid is 3.75 whereas acetic acid, with a methyl substituent, has a pKa of 4.76)

Deprotonation of carboxylic acids gives carboxylate anions; these are resonance stabilized, because the negative charge is delocalized over the two oxygen atoms, increasing the stability of the anion.Each of the carbon–oxygen bonds in the carboxylate anion has a partial double-bond character.The carbonyl carbon's partial positive charge is also weakened by the -1/2 negative charges on the 2 oxygen atoms.