Which Is More Acidic? (water)

pKa is the negative log of the acid dissociation constant (Ka). A lower pKa means a stronger acid that more readily donates a proton. Each unit decrease in pKa represents a 10-fold increase in acidity.

The compound with the lower pKa is the stronger acid. Compare pKa values directly: an acid with pKa 4 is 1,000 times stronger than one with pKa 7.

Water is a protic solvent that stabilizes anions through hydrogen bonding. DMSO is aprotic and does not stabilize anions as well. This means relative acidities can change between the two solvents, especially for charged acids.

Key factors: electronegativity of the atom bearing the negative charge, resonance stabilization of the conjugate base, inductive effects from nearby electronegative atoms, and the s-character of the orbital holding the lone pair.