Which option correctly represents the Henderson-Hasselbalch equation for pH?

Which option correctly represents the Henderson-Hasselbalch equation for pH?

• A. pKa = pH + log([acid]/[salt])
• B. pKa = pH + log([salt]/[acid])
• C. pH = pKa + log([acid]/[salt])
• D. pH = pKa + log([salt]/[acid])

Answer: (D)

The pH of a buffer is determined by how much weak acid is present relative to its conjugate base; this relationship is captured by relating pH to pKa and the ratio of base form to acid form. In the Henderson–Hasselbalch form, pH equals pKa plus the log of the ratio of the conjugate base (the salt form, [A-]) to the weak acid ([HA]). If you use [salt] for [A-] and [acid] for [HA], you get pH = pKa + log([salt]/[acid]). This arrangement makes intuitive sense: increasing the base form raises pH above pKa, decreasing the base form lowers pH below pKa, and when the two forms are equal, pH equals pKa. Other rearrangements would reverse the effect of changing the ratio and do not match the observed buffer behavior.

The pH of a buffer is determined by how much weak acid is present relative to its conjugate base; this relationship is captured by relating pH to pKa and the ratio of base form to acid form. In the Henderson–Hasselbalch form, pH equals pKa plus the log of the ratio of the conjugate base (the salt form, [A-]) to the weak acid ([HA]). If you use [salt] for [A-] and [acid] for [HA], you get pH = pKa + log([salt]/[acid]). This arrangement makes intuitive sense: increasing the base form raises pH above pKa, decreasing the base form lowers pH below pKa, and when the two forms are equal, pH equals pKa. Other rearrangements would reverse the effect of changing the ratio and do not match the observed buffer behavior.