Definition

Noun: - Equivalent weight: The atomic weight of an element that has the same combining capacity as a given weight of another element; the standard is 8 for oxygen. It is a measure of the mass of a substance that can combine with or displace one mole of hydrogen ions (H⁺) or electrons in a chemical reaction.

Usage

• Equivalent weight is a fundamental concept in stoichiometry and analytical chemistry, used to simplify calculations in reactions like acid-base titrations and redox reactions.
• It is calculated by dividing the molecular weight (or atomic weight) of a substance by its valence factor (n), which represents its combining capacity (e.g., number of hydrogen ions it can replace, donate, or accept, or electrons transferred).

Examples

• In an acid-base reaction, the equivalent weight of hydrochloric acid (HCl) is equal to its molecular weight (about 36.5 g/mol) because its valence factor (n) is 1 (it donates one H⁺ ion).
• For sulfuric acid (H₂SO₄), which can donate two H⁺ ions, the equivalent weight is half of its molecular weight (about 98 g/mol ÷ 2 = 49 g/equiv).
• In redox reactions, the equivalent weight of an oxidizing or reducing agent is its molecular weight divided by the number of electrons transferred per molecule.

Advanced Usage

• Gram equivalent weight: The mass in grams of a substance equal to its equivalent weight. It is a practical unit used in laboratory calculations.
  • One gram equivalent weight of sodium hydroxide (NaOH) neutralizes one gram equivalent weight of hydrochloric acid (HCl).
• Normality (N): A concentration unit defined as the number of gram equivalent weights of solute per liter of solution. It directly utilizes the concept of equivalent weight.
  • A 1 N solution of H₂SO₄ contains 49 grams of sulfuric acid per liter.

Variants and Related Words

• Equivalent (noun): Often used interchangeably with "equivalent weight" in context, especially when referring to the gram equivalent.
  • One equivalent of acid neutralizes one equivalent of base.
• Combining capacity: Refers to the valence or the power of an element to combine with other elements, which is central to determining equivalent weight.

Synonyms

• Gram equivalent
• Combining weight

Related Terms and Concepts

• Valence factor (n): The number used to divide the molecular weight to obtain the equivalent weight. It varies based on the type of reaction (acid-base, redox, precipitation).
• Milliequivalent: One-thousandth of an equivalent, commonly used in medical and biological contexts (e.g., electrolyte concentrations).
  • The patient's serum sodium level was measured in milliequivalents per liter (mEq/L).

Noun

1. the atomic weight of an element that has the same combining capacity as a given weight of another element; the standard is 8 for oxygen