Which statement correctly describes Ka and acid strength?

• A. Ka indicates acid strength; larger Ka means stronger acid.
• B. Ka equals the pH of the solution.
• C. Ka is only defined for bases.
• D. Ka is a constant that never changes with temperature.

Answer: (A)

The main idea here is how Ka reflects how readily an acid donates a proton in water. Ka is the acid dissociation constant for HA + H2O ⇌ H3O+ + A−, defined as Ka = [H3O+][A−]/[HA] at a given temperature. When Ka is larger, the equilibrium lies more toward the products, producing more hydronium ions and more conjugate base. That means the acid dissociates more, which is what we mean by a stronger acid. If Ka is small, the acid stays mostly undissociated, so it’s weaker.

Ka is not the same as pH; pH depends on [H3O+] but Ka is a ratio of concentrations at equilibrium. Also, Ka isn’t a universal constant—it changes with temperature because equilibrium positions shift with temperature. And Ka is defined for acids; bases have their own constant, Kb, for the corresponding base dissociation.