Which tube had a final pH of 6.21 when the buffer started at pH 7.0?

• A. Tube 2
• B. Tube 1
• C. Tube 3
• D. Tube 4

Answer: (B)

Buffers resist pH changes because they balance weak acid and its conjugate base. The pH of a buffered solution depends on the ratio of conjugate base to weak acid (pH ≈ pKa + log[A-]/[HA]). When you start at pH 7.0 and add acid, the pH drops as more HA forms and the ratio [A-]/[HA] decreases. How far the pH can fall depends on the buffer’s capacity—the total amount of buffering species present. A buffer with low capacity will show a larger pH drop for the same amount of added acid.

The final pH of 6.21 from a starting point of 7.0 means that particular tube faced a relatively strong acid challenge relative to its buffering capacity. In this set-up, that’s most consistent with the tube that had the least buffering capacity (or received more acid), which is why it ends at 6.21. The other tubes, with greater buffering capacity or less acid, would not drop as far.