A solution containing equal concentrations of a weak acid and its conjugate base has a pH of 5.45. What is the dissociation constant for the acid?
After the addition of a small amount of acid, a reasonable value of buffer pH would be 5,15.
If initially there are equal amounts of a weak acid and its conjugate base, the pH would be equal to the pKa, according to the Henderson-Hasselbach equation:
So, when adding a little amount of acid the pH should be only a little lower than the pKa. The value from the list that is a little lower than the pKa is 5.15