equivalence point

Meaning: At the equivalence point, the acid and base have reacted in exactly the ratio the balanced equation gives, so neither is in excess. The pH is set by what is left in solution. Strong acid with strong base leaves only a neutral salt and water, so pH = 7 at 25 °C. Weak acid with strong base leaves the conjugate base of the weak acid, which hydrolyzes and makes the pH above 7. Weak base with strong acid leaves the conjugate acid, so the pH is below 7. A polyprotic acid has one equivalence point for each proton removed.

Equation: naMaVa = nbMbVb, where M is molarity (mol/L), V is volume (L), and n is the number of H⁺ or OH⁻ each formula unit supplies (dimensionless).

On the MCAT: You may get a titration curve and be asked to find the equivalence point. It is the midpoint of the steep vertical rise. You may also be asked to calculate the titrant volume needed, or to choose an indicator whose color-change range includes the pH at equivalence. For a weak acid, the half-equivalence point (half the equivalence volume) is where pH = pKa.

Common confusion: Equivalence point is not the same as pH 7, and it is not the same as the endpoint. The equivalence point is defined by stoichiometry. The endpoint is where the indicator changes color, and a good indicator makes the two nearly coincide.