For a generic weak acid equilibrium, the acid dissociation constant Ka is defined as which expression?

For a generic weak acid equilibrium, the acid dissociation constant Ka is defined as which expression?

• A. Ka = [HA][H+]/[A-]
• B. Ka = [H+]/([A-][HA])
• C. Ka = [H+][A-]/[HA]
• D. Ka = [HA]/([H+][A-])

Answer: (C)

For a weak acid HA dissociating in water, the amount that dissociates is described by the equilibrium constant Ka for HA ⇌ H+ + A−. This constant is defined as the product of the concentrations of the two products divided by the concentration of the reactant: Ka = [H+][A−]/[HA]. In dilute solutions we usually approximate activities with concentrations, so Ka ≈ [H+][A−]/[HA]. This form directly reflects that at equilibrium, the tendency to form H+ and A− from HA is captured by the ratio of product concentrations to the reactant concentration. The other options mix up the placement of HA, H+, and A−, which would not represent the true equilibrium expression for this dissociation.

For a weak acid HA dissociating in water, the amount that dissociates is described by the equilibrium constant Ka for HA ⇌ H+ + A−. This constant is defined as the product of the concentrations of the two products divided by the concentration of the reactant: Ka = [H+][A−]/[HA]. In dilute solutions we usually approximate activities with concentrations, so Ka ≈ [H+][A−]/[HA]. This form directly reflects that at equilibrium, the tendency to form H+ and A− from HA is captured by the ratio of product concentrations to the reactant concentration. The other options mix up the placement of HA, H+, and A−, which would not represent the true equilibrium expression for this dissociation.