Equivalent weight

Jeremias Benjamin Richter

Titration

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Ion exchange resin beads

Equivalent weight is a concept used in chemistry to understand the mass of a substance that will combine with or displace a fixed amount of another substance. The term is particularly useful in the realms of analytical chemistry, stoichiometry, and chemical reactions, where it aids in the calculation of chemical equivalents.

Definition

Equivalent weight can be defined as the mass of a substance that reacts with or replaces 8.00 grams of oxygen, 1.008 grams of hydrogen, or 35.5 grams of chlorine. These values correspond to the atomic weights of these elements, reflecting their ability to combine with or displace other elements. The concept is rooted in the principle that elements have specific combining capacities, known as valences, which determine how they combine with other elements.

Calculation

The equivalent weight of a substance can be calculated using the formula:

\[ \text{Equivalent weight} = \frac{\text{Molecular weight}}{\text{Valence}} \]

where:

• Molecular weight is the sum of the atomic weights of all atoms in a molecule of the substance.
• Valence is the combining power of the substance's atom or ions, indicating how many atoms of hydrogen (or its equivalent) it can combine with or displace.

For acids, the equivalent weight is the mass of the acid that furnishes one mole of hydrogen ions (H+). For bases, it is the mass that can furnish one mole of hydroxide ions (OH-). For oxidizing and reducing agents, the equivalent weight is the mass that gains or loses one mole of electrons in a redox reaction.

Applications

Equivalent weight is a foundational concept in titration, where it is used to calculate the concentration of unknown solutions. By measuring the amount of a standard solution required to react completely with a given amount of the substance under analysis, chemists can determine the equivalent weight of the substance and, from that, its concentration in the solution.

Historical Context

The concept of equivalent weight dates back to the early 18th century, when chemists began to understand the fixed proportions by which elements combine to form compounds. This understanding laid the groundwork for the development of the law of definite proportions and the law of multiple proportions, which are central to modern chemistry.

Limitations

While the concept of equivalent weight is useful for calculations involving simple stoichiometric relationships, it has limitations. The advent of the mole concept and molar mass has largely superseded equivalent weight in many areas of chemistry, offering a more universal and precise means of quantifying substances in chemical reactions.

See Also

• Mole (unit)
• Stoichiometry
• Chemical reaction
• Analytical chemistry