Calculate pH of weak acids, weak bases, salts, and buffer solutions.
Buffer solutions resist changes in pH when small amounts of acid or base are added, making them essential in biological systems, industrial processes, and analytical chemistry. Understanding buffers requires connecting weak acid/base equilibrium to the Henderson-Hasselbalch equation, a core AP Chemistry Unit 8 topic and a recurring IChO calculation type.
The Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), is derived directly from the Ka expression and lets you calculate buffer pH without solving a full ICE table every time. It reveals the key insight: when [A-] = [HA] (equal moles of acid and conjugate base), pH = pKa exactly — this is the center of the buffer region, where the buffer resists pH change most effectively.
Buffer capacity — how much acid or base a buffer can absorb before its pH changes significantly — depends on the absolute concentrations of the acid/base pair, not just their ratio. A buffer made from 1 M acetic acid and 1 M acetate resists far more added strong acid than one made from 0.01 M concentrations, even though both start at the same pH.
Salt hydrolysis explains why solutions of salts like NH4Cl or NaCH3COO are not neutral. The ammonium ion (conjugate acid of a weak base) hydrolyzes water to produce H3O+, making NH4Cl solutions acidic; acetate ion (conjugate base of a weak acid) hydrolyzes to produce OH-, making sodium acetate solutions basic. The common ion effect further suppresses ionization of a weak acid or base when its conjugate is already present in solution, a principle that directly explains why buffers work.
This buffer and ionic equilibrium calculator lets you adjust acid/base concentrations, visualize the Henderson-Hasselbalch relationship in real time, and explore hydrolysis and common-ion scenarios — the exact calculation skill tested throughout AP Chemistry Unit 8 and IChO acid-base problems.