The first step toward answering this question is recognizing that there are two sources of the OAc - ion in this solution.
Sodium acetate, however, also dissociates in water to give the OAc - ion.
Thus, adding NaOAc reduces the extent to which HOAc dissociates. This mixture is therefore less acidic than 0.10 M HOAc, by itself. We can quantitate this discussion by setting up the problem as follows. We don't have any basis for predicting whether the dissociation of water can be ignored in this calculation, so let's make this assumption and check its validity later. C Substituting this information into the K a expression gives the following result.