100mL of a 1N H2SO4 solution

“N”有兩個意思。第一個意思是當量濃度，英文是 normality；第二個意思是當量濃度的單位，英文是 normal。

在 1 N 中的 “N”，是指當量濃度的單位（normal）。

在大約三十多年前，香港的中學會考化學課程中，溶液的濃度主要是用當量濃度（N），而不是今天的摩爾濃度（M）。

當量濃度(N) = 摩爾濃度(M) / 當量
即：摩爾濃度(M) = 當量濃度(N) x 當量

在酸鹼中和反應中，「當量」等於每摩爾該物質（酸或鹼）在中和反應中所涉及的 H^+ 或 OH^- 的摩爾數。由於 NaOH 中只含一個 OH^-，所以每摩爾 NaOH 在反應中只會涉及一摩爾 H+ 或 OH^-。
所以 1 N NaOH 的摩爾濃度 = 1 x 1 = 1 M

但有些情況下，同一物質的當量會在不同的反應中有所不同。以 H3PO4 為例：
在以下反應中，H3PO4的當量 = 1：
H3PO4 + NaOH → NaH2PO4 + H2O
在以下反應中，H3PO4的當量 = 2：
H3PO4 + 2NaOH → Na2HPO4 + 2H2O
在以下反應中，H3PO4的當量 = 3：
H3PO4 + 3NaOH → Na3PO4 + 3H2O

Normality highlights the chemical nature of salts: in solution, salts dissociate into distinct reactive species (ions such as H+, Fe3+, or Cl-). Normality accounts for any discrepancy between the concentrations of the various ionic species in a solution. For example, in a salt such as MgCl2, there are two moles of Cl- for every mole of Mg2+, so the concentration of Cl- as well as of Mg2+ is said to be 2 N (read: "two normal").

Definition:

A normal is one gram equivalent of a solute per liter of solution. The definition of a gram equivalent varies depending on the type of chemical reaction that is discussed - it can refer to acids, bases, redox species, and ions that will precipitate.

Usage:

It is critical to note that normality measures a single ion which takes part in an overall solute. For example, one could determine the normality of hydroxide or sodium in an aqueous solution of sodium hydroxide, but the normality of sodium hydroxide itself has no meaning. Nevertheless it is often used to describe solutions of acids or bases, in those cases it is implied that the normality refers to the H+ or OH− ion. For example, 2 Normal sulfuric acid (H2SO4), means that the normality of H+ ions is 2, or that the molarity of the sulfuric acid is 1. Similarly for 1 Molar H3PO4 the normality is 3 as it contains three H+ ions.