normality

Meaning: Normality counts reactive units instead of moles of formula units. One equivalent is the amount that supplies or consumes one mole of H⁺ (or OH⁻) in an acid-base reaction, or one mole of electrons in a redox reaction. Because it depends on what the solute does, the same solution can have different normalities in different reactions.

Equation: N = M × n, where N is normality (eq/L), M is molarity (mol/L), and n is equivalents per mole (eq/mol), the number of H⁺, OH⁻ or electrons exchanged per formula unit. At the equivalence point of a titration, N₁V₁ = N₂V₂, where V is volume (m³ in SI, but any one unit works if used for both).

On the MCAT: Titration passages give the volume and concentration of a standard acid or base and ask for an unknown concentration. Using normality skips the mole-ratio step. Typical cases: 0.10 M H₃PO₄ fully neutralized is 0.30 N; 0.10 M Ca(OH)₂ is 0.20 N; MnO₄⁻ reduced to Mn²⁺ gains 5 electrons, so 1 M is 5 N. When n is 1 or more, normality is equal to or greater than molarity.

Common confusion: Students treat normality as a fixed property of a solution, like molarity. It is not, because n depends on the reaction. H₃PO₄ gives 1, 2 or 3 eq/mol depending on how many protons are actually neutralized. Always ask what reaction is happening before converting M to N.