H2o

Now invert —log [HA]/[A ], which involves changing its sign, to obtain the Henderson-Hasselbalch equation:

Stated more generally, pH = pKa + log

[proton acceptor] [proton donor]

This equation fits the titration curve of all weak acids and enables us to deduce a number of important quantitative relationships. For example, it shows why the p^a of a weak acid is equal to the pH of the solution at the midpoint of its titration. At that point, [HA] equals [A—], and pH = pKa + log 1 = pKa + 0 = pKa

As shown in Box 2-3, the Henderson-Hasselbalch equation also allows us to (1) calculate pKa, given pH and the molar ratio of proton donor and acceptor; (2) calculate pH, given pKa and the molar ratio of proton donor and acceptor; and (3) calculate the molar ratio of proton donor and acceptor, given pH and pKa.

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