16.1: Normality

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Normality expresses concentration in terms of the equivalents of one chemical species that react stoichiometrically with another chemical species. Note that this definition makes an equivalent, and thus normality, a function of the chemical reaction. Although a solution of H₂SO₄ has a single molarity, its normality depends on its reaction.

We define the number of equivalents, n, using a reaction unit, which is the part of a chemical species that participates in the chemical reaction. In a precipitation reaction, for example, the reaction unit is the charge of the cation or the anion that participates in the reaction; thus, for the reaction

\[\ce{Pb^{2+}}(aq) + 2\ce{I-}(aq) \ce{<=>} \ce{PbI2}(s) \nonumber\]

n = 2 for Pb²⁺ and n = 1 for I^–. In an acid–base reaction, the reaction unit is the number of H⁺ ions that an acid donates or that a base accepts. For the reaction between sulfuric acid and ammonia

\[\ce{H2SO4}(aq) + 2\ce{NH3}(aq) \ce{<=>} 2\ce{NH4+}(aq) + \ce{SO4^{2-}} \nonumber\]

n = 2 for H₂SO₄ because sulfuric acid donates two protons, and n = 1 for NH₃ because each ammonia accepts one proton. For a complexation reaction, the reaction unit is the number of electron pairs that the metal accepts or that the ligand donates. In the reaction between Ag⁺ and NH₃

\[\ce{Ag+}(aq) + 2\ce{NH3}(aq) \ce{<=>} \ce{Ag(NH3)2+}(aq) \nonumber\]

n = 2 for Ag⁺ because the silver ion accepts two pairs of electrons, and n = 1 for NH₃ because each ammonia has one pair of electrons to donate. Finally, in an oxidation–reduction reaction the reaction unit is the number of electrons released by the reducing agent or accepted by the oxidizing agent; thus, for the reaction

\[2\ce{Fe^{3+}}(aq) + \ce{Sn^{2+}}(aq) \ce{<=>} \ce{Sn^{4+}}(aq) + 2\ce{Fe^{2+}}(aq) \nonumber\]

n = 1 for Fe³⁺ and n = 2 for Sn²⁺. Clearly, determining the number of equivalents for a chemical species requires an understanding of how it reacts.

Normality is the number of equivalent weights, EW, per unit volume. An equivalent weight is the ratio of a chemical species’ formula weight, FW, to the number of its equivalents, n.

\[EW = \frac {FW} {n} \nonumber\]

The following simple relationship exists between normality, N, and molarity, M,

\[N = n \times M \nonumber\]