8.7 pH and pKa

Henderson-Hasselbalch equation

For a buffer solution containing a weak acid and its conjugate base, pH = pKa + log([A⁻]/[HA]). This relates the pH of the solution to the acid's pKa and the ratio of conjugate base to weak acid concentrations. It's most accurate near the equivalence region of weak acid/base equilibria.

Half-equivalence point

At the half-equivalence point of a titration, exactly half the weak acid has been converted to its conjugate base, so [HA] = [A⁻]. Since log(1) = 0, the Henderson-Hasselbalch equation simplifies to pH = pKa. This is a quick way to find pKa from a titration curve.

pKa and acid strength

pKa = -log(Ka). A smaller pKa means a larger Ka, indicating a stronger acid that dissociates more. Comparing pKa values lets you quickly rank relative acid strengths — the acid with the lowest pKa is the strongest among weak acids being compared.

Key terms

Henderson-Hasselbalch equation

pH = pKa + log([A⁻]/[HA]); relates pH of a buffer to pKa and the ratio of conjugate base to weak acid.

Half-equivalence point

The point in a titration where [HA] = [A⁻], so pH = pKa.

pKa definition

pKa = -log(Ka); a measure of acid strength — smaller pKa means stronger acid.

Effect of [A⁻] > [HA] on pH

pH will be greater than pKa, since the log term becomes positive.

Effect of [HA] > [A⁻] on pH

pH will be less than pKa, since the log term becomes negative.

Try an AP-style question

An acid has pKa = 4.8 and the solution pH is 3.0. Which form dominates?

1. Equal amounts of both
2. The deprotonated form, A⁻
3. The protonated form, HA
4. Neither form