Weak Acids and Ka

Partial Dissociation

When a weak acid HA dissolves in water, only a small fraction of its molecules donate a proton to water:

HA(aq) + H₂O(l) ⇌ A⁻(aq) + H₃O⁺(aq)

Or more simply:

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

At equilibrium, the solution contains a mixture of undissociated HA molecules, H⁺ ions, and A⁻ ions. For a typical weak acid like ethanoic acid (Ka = 1.74 × 10⁻⁵ mol dm⁻³), fewer than 1% of the molecules are dissociated in a 1.0 mol dm⁻³ solution.

---

The Acid Dissociation Constant, Ka

The equilibrium constant for the dissociation of a weak acid is called the acid dissociation constant, Ka:

Ka = [H⁺][A⁻] / [HA]

The units of Ka are mol dm⁻³ (since there are two concentration terms on top and one on the bottom). Water does not appear in the expression because its concentration is effectively constant.

Ka tells you the strength of a weak acid:

• Larger Ka → more dissociation → stronger (within the category of weak acids)
• Smaller Ka → less dissociation → weaker