General ideas about buffers

When Sodium Fluoride dissolves in water, the reaction goes to completion, thus we obtain:

NaF(aq)+H2O(l) →Na+(aq)+F−(aq)

Since Na+ is the conjugate of a strong base, it will have no effect on the pH or reactivity of the buffer. The addition of NaF to the solution will, however, increase the concentration of F- in the buffer solution, and, consequently, by Le Chatlier's principle, lead to slightly less dissociation of the HF in the previous equilibrium, as well. The presence of significant amounts of both the conjugate acid, HF, and the conjugate base, F, allows the solution to function as a buffer.