In a buffer made from a weak acid HA and its conjugate base A-, adding A- will shift pH how?

• A. Decrease pH
• B. Increase pH
• C. No change
• D. pH fluctuates unpredictably

Answer: (B)

In this buffer, pH is governed by Henderson–Hasselbalch: pH = pKa + log([A-]/[HA]). Adding more A- increases the ratio [A-]/[HA], so the log term becomes larger and the pH rises. The system adjusts by shifting the equilibrium toward HA to rebalance, which also helps reduce acidity (lower H+ concentration), reinforcing the pH increase. The buffer resists big changes, so the shift in pH is modest rather than unpredictable.