For a weak acid HA ⇌ H+ + A-, which expression correctly represents Ka?

For a weak acid HA ⇌ H+ + A-, which expression correctly represents Ka?

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

Answer: (B)

This question tests how the acid dissociation constant is defined for a weak acid. For HA ⇌ H+ + A-, the equilibrium constant expression is the ratio of the product concentrations to the reactant concentration: Ka = [H+][A-] / [HA]. This reflects the balance between dissociation and recombination at equilibrium. In a weak acid, only a small fraction dissociates, so [HA] remains relatively large and Ka ends up being less than 1. Why this form is correct: Ka must incorporate both products, since dissociation creates H+ and A-, and it must include the undissociated acid in the denominator because that’s the remaining reactant that determines how far the reaction proceeds. The other forms would either omit one of the necessary species, place [HA] in the numerator, or multiply all species together, none of which align with how equilibrium constants are defined.

This question tests how the acid dissociation constant is defined for a weak acid. For HA ⇌ H+ + A-, the equilibrium constant expression is the ratio of the product concentrations to the reactant concentration: Ka = [H+][A-] / [HA]. This reflects the balance between dissociation and recombination at equilibrium. In a weak acid, only a small fraction dissociates, so [HA] remains relatively large and Ka ends up being less than 1.

Why this form is correct: Ka must incorporate both products, since dissociation creates H+ and A-, and it must include the undissociated acid in the denominator because that’s the remaining reactant that determines how far the reaction proceeds. The other forms would either omit one of the necessary species, place [HA] in the numerator, or multiply all species together, none of which align with how equilibrium constants are defined.