Hydrofluoric acid, HF, is a weak acid. Which of the following is the correct expression for the K_a of hydrofluoric acid? A) [H₃O⁺][HF]/[F⁻] B) [HF]/[ H₃O⁺][F⁻] C) [H₃O⁺][F⁻]/[HF] D) [HF][F⁻]/[ H₃O⁺]

Question

Quizplus · Accepted Answer

The Ka of a weak acid is equal to the concentration of the products (H+ and A-) over the concentration of the reactants (HA). In the case of HF: HF + H2O ⇌ H3O+ + F- Ka = [H3O+][F-]/[HF] However, the concentration of H3O+ in a dilute HF solution is equal to the concentration of HF, as HF is the predominant acid species in solution. Therefore: Ka = [HF][F-]/[HF] = [F-] The expression for the K_a of hydrofluoric acid is [F-], which corresponds to choice C.

Hydrofluoric Acid, HF, Is a Weak Acid