Short Answer

Equivalent weight is the mass of a substance that reacts with or supplies one mole of hydrogen ions (H⁺), one mole of electrons, or another standard amount depending on the type of reaction. It helps compare the reactive capacity of different substances.

Equivalent weight is widely used in acid–base reactions, redox reactions, and titrations. It tells how much of a substance is needed to react completely with another substance in a chemical process.

Detailed Explanation

Equivalent Weight

Equivalent weight is an important concept in chemistry that expresses the effective reactive capacity of a substance. It represents the mass of a substance that reacts chemically with a fixed quantity of another standard substance. Traditionally, this standard is:

• 1 mole of hydrogen ions (H⁺) in acid–base reactions
• 1 mole of electrons in redox reactions
• 1 mole of hydroxide ions (OH⁻) for bases
• A defined quantity of another substance in precipitation or other reactions

Equivalent weight allows chemists to compare substances based on how much of them is required to react completely in a given chemical process. It simplifies calculations in titration, stoichiometry, and solution preparation.

General formula for equivalent weight

Although the meaning of equivalent weight changes slightly depending on the type of reaction, the general formula remains:

Equivalent weight = Molar mass ÷ n

Where n depends on:

• Number of H⁺ ions donated or accepted (acids/bases)
• Number of electrons gained or lost (redox reactions)
• Valency or charge of the ion (salts)

This value helps determine the amount of a substance needed to produce or react with one equivalent of another reacting substance.

Equivalent weight in different types of reactions

Equivalent weight varies because different reactions involve different reacting units.

1. Equivalent weight of acids

For acids, equivalent weight = molar mass ÷ basicity.

Basicity is the number of hydrogen ions (H⁺) an acid can release.

Examples:

• HCl releases 1 H⁺ → basicity = 1
• H₂SO₄ releases 2 H⁺ → basicity = 2
• H₃PO₄ releases 3 H⁺ → basicity = 3

So, H₂SO₄ has half the equivalent weight of its molar mass because it supplies 2 H⁺ ions.

2. Equivalent weight of bases

For bases, equivalent weight = molar mass ÷ acidity.

Acidity is the number of hydroxide ions (OH⁻) a base can provide.

Examples:

• NaOH provides 1 OH⁻ → acidity = 1
• Ca(OH)₂ provides 2 OH⁻ → acidity = 2

This helps determine how much base is needed to neutralize a given acid.

3. Equivalent weight in redox reactions

In redox reactions, equivalent weight = molar mass ÷ number of electrons gained or lost.

Example:
Fe²⁺ → Fe³⁺ loses 1 electron → n = 1
MnO₄⁻ in acidic medium gains 5 electrons → n = 5

Thus, equivalent weight changes depending on oxidation state change.

4. Equivalent weight of salts

For salts, the equivalent weight is:

Equivalent weight = molar mass ÷ total positive or negative charge

Example:

• NaCl has charge 1 → n = 1
• CaCl₂ has charge 2 for Ca²⁺ → n = 2

This helps in precipitation reactions and salt titrations.

Uses of equivalent weight

Equivalent weight has many important uses in chemistry:

1. Titration calculations

It helps compute normality (N), which is based on equivalents:

Normality = equivalents of solute ÷ volume of solution (in L)

2. Preparing standard solutions

Chemists use equivalent weight to prepare accurately measured solutions for reactions.

3. Redox analysis

Knowing equivalent weight helps determine electron transfer amounts.

4. Acid–base neutralization

It helps identify how much acid is needed to neutralize a base and vice versa.

5. Precipitation reactions

Equivalent weight helps calculate how much salt participates in forming a precipitate.

Examples to understand equivalent weight better

Example 1: Equivalent weight of H₂SO₄

Molar mass = 98 g/mol
Basicity = 2
Equivalent weight = 98 ÷ 2 = 49 g/equivalent

Example 2: Equivalent weight of NaOH

Molar mass = 40 g/mol
Acidity = 1
Equivalent weight = 40 ÷ 1 = 40 g/equivalent

Example 3: Equivalent weight of KMnO₄ in acidic medium

Molar mass = 158 g/mol
Electrons gained = 5
Equivalent weight = 158 ÷ 5 = 31.6 g/equivalent

These examples show that equivalent weight depends on how a substance reacts, not just its molar mass.

Why equivalent weight is important

Equivalent weight is useful because it:

• Simplifies chemical calculations
• Helps compare different reacting substances
• Makes stoichiometric calculations easier
• Connects molarity, normality, and titration methods
• Allows accurate preparation of solutions for laboratory or industrial use

Although modern chemistry often uses molarity instead of normality, equivalent weight remains essential in many practical applications.

Conclusion

Equivalent weight is the mass of a substance that reacts with or supplies one mole of H⁺ ions, OH⁻ ions, electrons, or other defined reactive units. It depends on the substance’s behaviour in a reaction and is calculated using molar mass and the number of reactive units involved. Equivalent weight is important in titrations, redox reactions, acid–base chemistry, and solution preparation. Understanding this concept makes stoichiometric and analytical calculations easier and more accurate.