titration

Meaning: A titration finds the concentration of an analyte by adding a titrant of known concentration, usually from a buret, until the reaction is stoichiometrically complete. That is the equivalence point. An indicator color change or a pH meter marks the endpoint, which is chosen to fall near the equivalence point. When a weak acid is titrated with a strong base, the early curve is a buffer region, and at the half-equivalence point pH = pKa.

Equation: (H⁺ per acid) × Ma × Va = (OH⁻ per base) × Mb × Vb, where M is molarity (mol/L) and V is volume (L).

On the MCAT: A passage gives a curve of pH versus volume added. You locate the equivalence point at the steepest rise, read pKa at the half-equivalence point, pick an indicator whose transition range includes the equivalence-point pH, or calculate an unknown concentration from the volumes. Polyprotic acids give more than one equivalence point.

Common confusion: The equivalence point is not always at pH 7. That holds for a strong acid with a strong base at 25 °C. A weak acid titrated with a strong base ends above 7 because its conjugate base hydrolyzes water, and a weak base titrated with a strong acid ends below 7. Also, the endpoint (indicator change) is close to, but not the same as, the equivalence point (mole match).