What normality is
Normality (N) is the number of equivalents of a solute per liter of solution. An equivalent is the amount that supplies or reacts with one mole of the species that matters in the reaction: H⁺ ions in acid-base chemistry, OH⁻ ions for bases, or electrons in redox chemistry.
Normality = molarity × number of equivalents per mole
How to calculate it
- Moles of solute = mass ÷ molar mass.
- Molarity = moles ÷ volume in liters.
- Normality = molarity × equivalents per mole.
- Equivalent weight = molar mass ÷ equivalents per mole.
Example
Dissolve 4.9 g of sulfuric acid (H₂SO₄, 98.08 g/mol) in water to make 500 mL of solution:
- moles = 4.9 ÷ 98.08 ≈ 0.050 mol;
- molarity = 0.050 ÷ 0.5 ≈ 0.1 mol/L;
- H₂SO₄ releases 2 H⁺ per molecule, so normality ≈ 0.2 N;
- equivalent weight = 98.08 ÷ 2 = 49.04 g/eq.
Equivalents per mole for common reagents
| Substance | Reaction | Equivalents per mole |
|---|---|---|
| HCl, HNO₃ | acid | 1 |
| H₂SO₄ | acid | 2 |
| H₃PO₄ | acid, full neutralization | 3 |
| NaOH, KOH | base | 1 |
| Ca(OH)₂ | base | 2 |
| KMnO₄ | oxidant in acid (Mn⁷⁺ → Mn²⁺) | 5 |
| K₂Cr₂O₇ | oxidant in acid | 6 |
The same compound can have different normalities depending on the reaction. Phosphoric acid titrated only to its first endpoint gives 1 equivalent per mole, not 3.
Why use normality?
At the equivalence point of a titration, equivalents of acid equal equivalents of base, so:
N₁ × V₁ = N₂ × V₂
This makes calculations quick when acids and bases have different numbers of reactive groups. Normality is still common in water analysis, such as alkalinity and hardness titrations, and in older laboratory procedures.
Limits
IUPAC discourages normality because it depends on the reaction. Modern practice prefers molarity with the stoichiometry written out. Always state the reaction when you report a normality.