15.07: Normality

Normality expresses concentration in terms of the equivalents of one chemical species reacting 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 participating in the chemical reaction. In a precipitation reaction, for example, the reaction unit is the charge of the cation or anion participating in the reaction; thus, for the reaction

$$\ce{Pb}^{2+}(aq) + \ce{2 I}^- (aq) \rightleftharpoons \ce{PbI}_2(s)$$

n = 2 for Pb²⁺(aq) and n = 1 for 2 I^-(aq). 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

$$\ce{H_2SO_4}(aq) + \ce{2NH_3}(aq) \rightleftharpoons \ce{2NH_4^+}(aq) + \ce{SO}_4^{2-}(aq)$$

n = 2 for H₂SO₄(aq) because sulfuric acid donates two protons, and n = 1 for NH₃(aq) 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

$$\ce{Ag^+}(aq) + \ce{2NH_3}(aq) \rightleftharpoons \ce{Ag(NH_3)2+}(aq)$$

n = 2 for Ag⁺(aq) 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

$$\ce{2Fe}^{3+}(aq) + \ce{Sn}^{2+}(aq) \rightleftharpoons \ce{Sn}^{4+}(aq) + \ce{2Fe}^{2+}(aq)$$

$n = 1$ for $\ce{Fe^3+}(aq)$ and $n = 2$ for $\ce{Sn^2+}(aq)$. 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, $n$.

$$EW = \dfrac{FW}{n}$$

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

$$N = n \times M$$

Contributors and Attributions

David Harvey (DePauw University)

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