neutralize

Meaning: Neutralization is the reaction of an acid with a base in stoichiometrically equivalent amounts, so the moles of H⁺ available equal the moles of OH⁻ (or other proton acceptor). A strong acid and strong base give a salt and water, as in HCl + NaOH → NaCl + H₂O. The solution is pH 7 (at 25 °C) only when neither ion of the salt reacts with water. If a weak acid is neutralized by a strong base, the leftover conjugate base accepts protons from water, so the pH is above 7. A weak base neutralized by a strong acid leaves a conjugate acid, so the pH is below 7.

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 can supply (dimensionless). Subscript a is the acid and b is the base.

On the MCAT: Titration passages ask for the volume of base needed to reach the equivalence point, the concentration of an unknown acid, or the pH at equivalence. You may also read a titration curve. For a weak acid, the half-equivalence point has pH = pKa, and the equivalence point lies above 7. Polyprotic acids need n greater than 1.

Common confusion: Students equate neutralized with neutral. Neutralization means equal moles of acid and base have reacted. The pH then depends on the salt formed, so identify whether its ions are conjugates of weak acids or weak bases.