Equivalent concentration

Equivalent concentration (also known as normality) is a measure used in chemistry to express the concentration of solutes in a solution. It is denoted by the symbol N and is defined as the number of equivalents of solute per liter of solution. The concept of equivalent concentration is particularly useful in the context of acid-base chemistry, redox reactions, and precipitation reactions, where the stoichiometry involves moles of ions or electrons rather than moles of compounds.

Definition

The equivalent concentration, N, of a solution is calculated using the formula:

\[N = \frac{{\text{Number of equivalents of solute}}}

\]

An equivalent (Eq) is the amount of a substance that reacts with, or is equivalent to, an arbitrary amount of another substance in a given chemical reaction. The concept of equivalents is based on the fact that in a chemical reaction, ions, molecules, or electrons are transferred in fixed proportions.

Calculation of Equivalents

The number of equivalents depends on the type of reaction being considered. For acids and bases, an equivalent is defined as the amount of acid or base that can donate or accept one mole of protons (H+ ions). For oxidation-reduction (redox) reactions, an equivalent is the amount of a substance that can donate or accept one mole of electrons.

Acid-Base Reactions

In acid-base reactions, the equivalent weight of an acid is its molar mass divided by its basicity (number of H+ ions it can donate), and for a base, it is its molar mass divided by its acidity (number of H+ ions it can accept).

Redox Reactions

In redox reactions, the equivalent weight of a substance is its molar mass divided by the number of electrons it can donate or accept, which is determined by its oxidation state change in the reaction.

Applications

Equivalent concentration is widely used in titration calculations to determine the concentration of unknown solutions. It simplifies the calculations by directly relating the volume and concentration of the titrant and analyte through the concept of equivalents.

Limitations

While the concept of equivalent concentration is useful in many scenarios, it has limitations. It assumes that all reactions are complete and stoichiometric, which may not be the case in real-world applications. Additionally, the concept of equivalents can become complex in multi-step reactions or when dealing with polyprotic acids and bases.

See Also

• Molarity
• Molality
• Mass concentration
• Volume concentration