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

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Study for the IMAT Chemistry Exam. Access flashcards and multiple choice questions; hints and explanations included for each question. Prepare effectively for your exam.

Multiple Choice

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

Explanation:
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.

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