Buffer Response to Strong Acid

This animation represents a simplified molecular view or a model for what happens when a small amount of strong acid is added to a buffer. The major species in the buffer solution are HA and A^- and the added hydronium ion (the limiting reactant) reacts with the weak base component of the buffer producing the conjugate acid:
A^-(aq) + H₃O⁺(aq) ---> HA(aq) + H₂O(l)
If the pK_a of the weak acid is 5.00, and each molecule or ion represents one mole, then the pH is 4.57 where the hydronium ion in the solution simply represents the acidic nature of the buffer. The pH of the original buffer was 4.92 and the small change in pH reflects a change in the ratio of [A^-]/[HA].