Which of the following would have the highest buffer capacity?

0.35 M HCOOH / 0.35M HCOO-

1.25 M HCO3 - / 1.25 M CO32-

0.500 M NH4 + / 0.500 M NH3

1.0 M HCN / 1.0 M CN-

0.40 M HF / 0.40 M F-

1.25 M HCO₃⁻ / 1.25 M CO₃²⁻

Explanation:

Buffer capacity refers to the amount of a strong acid or base required per liter of the buffer to change its pH by one. This amount is directly related to the concentration of the conjugate acid-base pair in the buffer since the buffer pair neutralizes the strong acid or base.

Thus, the highest buffer capacity is found in the solution that has the highest concentration of the conjugate acid-base pair, which is 1.25 M HCO₃⁻ / 1.25 M CO₃²⁻.