For a weak acid HA, which constant describes its acid strength?

For a weak acid HA, which constant describes its acid strength?

• A. Ka
• B. Kb
• C. Kw
• D. Ksp

Answer: (A)

Acid strength is quantified by the acid dissociation constant for HA. For the equilibrium HA ⇌ H+ + A−, Ka = [H+][A−]/[HA]. A larger Ka means more HA dissociates in water, so the acid is stronger; a smaller Ka means less dissociation, so it is weaker. The related concept, pKa = −log Ka, shows that smaller pKa corresponds to stronger acid. The other constants don’t describe acid strength: Kb is the base dissociation constant for the conjugate base, Kw is the autoionization constant of water, and Ksp is the solubility product for sparingly soluble salts.

Acid strength is quantified by the acid dissociation constant for HA. For the equilibrium HA ⇌ H+ + A−, Ka = [H+][A−]/[HA]. A larger Ka means more HA dissociates in water, so the acid is stronger; a smaller Ka means less dissociation, so it is weaker. The related concept, pKa = −log Ka, shows that smaller pKa corresponds to stronger acid.

The other constants don’t describe acid strength: Kb is the base dissociation constant for the conjugate base, Kw is the autoionization constant of water, and Ksp is the solubility product for sparingly soluble salts.