What does the acid dissociation constant Ka measure?

• A. The strength of an acid in solution.
• B. The rate at which the acid donates a proton.
• C. The concentration of the acid at equilibrium.
• D. The pH of the solution.

Answer: (A)

Ka tells you how far an acid dissociates in water at equilibrium. For the dissociation HA + H2O ⇌ H3O+ + A−, Ka = [H3O+][A−]/[HA]. A larger Ka means more of the acid splits into ions, so the solution is more acidic and the acid is considered stronger. Ka is an equilibrium constant and depends on temperature, so changing temperature changes the extent of dissociation. This differs from a rate; Ka describes where the system lies at equilibrium, not how fast protons are donated. It also doesn’t directly give the pH, since pH depends on the actual hydronium concentration in the solution, which reflects both Ka and the starting amount of acid. It isn’t simply the concentration of the acid at equilibrium, but a ratio of concentrations of products to the undissociated acid.