General ideas about buffers

If acid is added:

F−(aq)+H3O+(aq) ⇌ HF(aq)+H2O(l)

In this reaction, the conjugate base, F, will neutralize the added acid, H3O+ , and this reaction goes to completion, because the reaction of F- with H3O+ has an equilibrium constant much greater than one. (In fact, the equilibrium constant the reaction as written is just the inverse of the Ka for HF: 1/Ka(HF) = 1/(6.6x10-4 ) = 1.5x10+3.) So long as there is more F- than H3O+ , almost all of the H3O+ will be consumed and the equilibrium will shift to the right, slightly increasing the concentration of HF and slightly decreasing the concentration of F, but resulting in hardly any change in the amount of H3O+ present once equilibrium is re-established.