8 julio, 2024

Equivalent weight: what it is, formula, what it is for, calculation, examples

What is the equivalent weight?

He equivalent weight is the weight or mass of one equivalent of substance. That is, it is the weight or mass of a substance that is chemically equivalent to 1 mol of another substance of the same type that is used as a reference standard and is considered normal.

The normal substance that is taken as a reference depends on the type of chemical reaction involved and the role that the compound plays in that reaction. Normal compounds always react with each other with 1:1 stoichiometry.

For example, if we are talking about an acid/base reaction, the equivalent weight of an acid would be the weight of said compound that is equivalent to 1 mol of a normal acid, which is one that has a single ionizable proton.

Before it was known that matter was made up of atoms and molecules, a series of laws had already been established that determined the mass (or weight) ratio between the reactants and products of a chemical reaction.

These laws formed the basis of stoichiometric calculations during the early days of chemistry as a science.

The concept of equivalent weight is related to this early stage in the development of chemistry. Next, we will discuss how it is calculated, what it is for, and some common examples of its calculation.

Formula and units of equivalent weight

The equivalent weight is calculated from the molecular weight of a substance using the following formula:

where PE is the equivalent weight, PM is the molecular weight or molar mass of the substance, and w represents the number of equivalents per mole.

The latter always consists of an integer equal to or greater than 1. It represents the number of molecules of a “normal” reactant to which each molecule of the substance is equivalent. Therefore, it depends on the type of reagent in question and the type of chemical reaction involved.

It can also be calculated from the mass, m, and the number of equivalents:

units of equivalent weight

As can be seen from the formula above, the equivalent weight is expressed in units of mass (usually grams) over number of equivalents. That is, the units are g/eq.

The value of w in the first formula above it is expressed in units of equivalents per mol (eq/mol), while the molar mass is expressed in grams per mol (g/mol).

What is the equivalent weight used for?

The main utility of the equivalent weight is that it simplifies stoichiometric calculations. The equivalent weight of one substance is equal to the equivalent weight of another substance of the same type and for the same kind of chemical reaction. In fact, that’s where it gets its name from.

This allows us to carry out stoichiometric calculations very easily and without the need to write and balance chemical equations.

How is the equivalent weight calculated?

As we saw before, the equivalent weight is calculated from the molecular weight and the number of equivalents per mole or w.

Molecular Weight Calculation

The molecular weight of a substance is determined by adding the atomic weight of all the atoms that make it up. These atomic weights are found in the periodic table.

Determination of w

The value of w for a substance depends on the type of reaction in which it is involved. That is, it depends on the type of reagent in question. For each type of substance there is a different way of determining w.

The following table shows how to determine w for different cases.

reagent type
Value of w

Acids Number of ionizable hydrogens per molecule of acid. Bases Number of protons that can capture or neutralize each molecule of the base. Oxidizing agents Number of electrons captured by each mole of oxidizing agent in the reduction half-reaction. Reducing Agents Number of electrons released by each mole of reducing agent in the oxidation half-reaction. Salts Total number of positive or negative charges generated when one mole of salt is ionized. neutral substances 1

It should be noted that, if a substance can participate in more than one type of chemical reaction, it may have more than one value of w and therefore more than one equivalent weight.

Importance and applications of the equivalent weight

The equivalent weight is used to calculate the number of equivalents of a chemical substance. This number of equivalents is always equal to the number of equivalents of another substance with which it reacts.

This is mainly used in two different scopes:

in analytical chemistry

Equivalents are often used to facilitate stoichiometric calculations in analytical chemistry. This is particularly true in the case of volumetric and gravimetric analysis.

Knowing the volume of the titration at the equivalence point and the concentration of the titrant, as well as the equivalent weight of the titrate, allow one to easily calculate the mass of the analyte present in a sample.

in electrolysis

In the field of electrolysis, the equivalent weight represents the weight of a metal or other substance that can be produced by passing 96,500 C of charge through the electrolytic cell. This facilitates stoichiometric and economic calculations of the electrolysis process.

Examples of Equivalent Weight Calculation


The atomic weight of calcium is 40 and this metal forms cations with valence +2, so its equivalent weight is 40/2 = 20 g/eq.

sulfuric acid (H2SO4)

The molecular weight of this compound is 98 and it is a diprotic acid, so w = 2. Therefore, the equivalent weight is PE: 98/2 = 49 g/eq.

Aluminum hydroxide (Al(OH)3)

Aluminum hydroxide (MW = 78) is an amphoteric compound that can act as both an acid and a base.

If it acts as a base releasing its three hydroxides, then w = 3, so PE: 78/3 = 26 g/eq. Instead, when it acts as an acid, it picks up a water molecule and releases a proton, so in this case, w = 1 and PE = 78 g/eq.

Calcium phosphate (Ca3(PO4)2)

Calcium phosphate is a salt that, when dissociated, produces 3 Ca2+ ions and 2 PO43- ions.

In either case, the number of like charges produced by each unit of calcium phosphate is 6, so w = 6 and PE = 310/6 = 51.67 g/eq.


Universal Encyclopedia (nd). equivalent weight. Taken from enciclopedia_universal.es-academic.com. Chemistry, Inorganic Chemistry (nd). Equivalent Weight. Taken from fullquimica.com. Chemistry.ES (sf). Equivalent. Taken from quimica.es. Spiegato (2021). What is the equivalent weight? Taken from spiegato.com.

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