Final answer:
After adding a small amount of acid to a buffer with equal amounts of weak acid and its conjugate base at pH 5.20, the reasonable value of buffer pH would be 5.10.
Step-by-step explanation:
If a buffer contains equal amounts of a weak acid and its conjugate base and has a pH of 5.20, the addition of a small amount of acid will slightly lower the pH. Since a buffer system resists drastic changes in pH, the reasonable value of buffer pH after the addition of a small amount of acid would be 5.10. The pH level will not fall to 6.30 or rise to 4.10, and 5.30 is higher than the initial pH making it an unlikely outcome after adding an acid. The Henderson-Hasselbalch equation helps us understand that the addition of a small amount of acid to a buffer already at its maximum pH control would result in a minimal change, keeping the pH close to the pKa of the weak acid.