All About Normality

Knowing how many moles of solute are present in one litre of solution is frequently useful, especially when these solutions are used in chemical processes. Normality and molarity help us just with that. Also, while pH is used to evaluate the acidity or alkalinity of natural liquids, normality is used in the lab to indicate the concentration of the considerably stronger acid and base solutions. Normality is based on molarity, but it also considers a property of acids and bases known as “equivalents,” which we shall discuss in the following section. Let us first define normality. 

What is normality? 

In simple words, we can define normality as the gramme equivalent weight of solute per litre of solution in a chemical solution. Equivalent concentration is another name for normality. The sign “N” or “eq/L” is used to indicate it (equivalents per litre). To calculate the gramme equivalent weight, you must first determine how many hydrogen ions (H+ or H3O+), hydroxide ions (OH–), or electrons (e–) are transported in a reaction, as well as the chemical species’ valence.

The concentration of a hydrochloric acid solution, for example, maybe written as 0.1 N HCl. A gramme equivalent weight or equivalent is a measure of a chemical species’ reactive capability (ion, molecule, etc.). The molecular weight and valence of the chemical species are used to calculate the equivalent value. The only concentration unit that is response-dependent is normality.

What is equivalent weight?

The equivalent weight of a material is defined as the ratio of its molar mass to its valence. Equivalence factor is another term for valence. The valence of an acid, a base, or salt is the number of H+, OH–, or charge present in ionic forms. The amount of electrons that an oxidising or reducing substance may take or contribute is quantified as the valence or equivalency factor in reduction/oxidation (redox) processes. It’s termed Gram Equivalent Weight when the Equivalent Weight is stated in grams using molar mass in grams.

We use a reaction unit to specify the number of equivalents, n, which is the part of a chemical species that contributes to the chemical reaction. In a precipitation reaction, for instance, the reaction unit is the charge of the cation or anion that participates in the reaction; hence, the reaction unit is the charge of the cation or anion that participates in the reaction.

Pb2+(aq)+2I−(aq)↽⇀PbI2(s)

Pb2+ has n = 2 and I– has n = 1. 

The amount of H+ ions that an acid produces or absorbs is the reaction unit in an acid-base reaction. For instance, for a reaction involving Sulphuric acid and ammonia,

H2SO4(aq)+2NH3(aq)↽⇀2NH+4(aq)+SO2−4

Because sulfuric acid contributes to two protons, n = 2 for H2SO4 and n = 1 for NH3, each ammonia absorbs one proton. 

The number of electron pairs that the metal takes or that the ligand contributes is the reaction unit in a complexation reaction. For the reaction below,

Ag+(aq)+2NH3(aq)↽⇀Ag(NH3)+2(aq)

Because the silver ion receives two pairs of electrons, n = 2 for Ag+ and n = 1 for NH3, each ammonia has two electrons to give. 

Finally, the amount of electrons emitted by the reducing agent or received by the oxidising agent is the reaction unit in an oxidation-reduction process. For instance, for the following reaction,

2Fe3+(aq)+Sn2+(aq)↽⇀Sn4+(aq)+2Fe2+(aq)

For Fe3+, n = 1 and for Sn2+, n = 2. 

Clearly, knowing how a chemical species respond is necessary for estimating the number of equivalents. The number of equivalent weights (EW) per unit volume is the measure of normality. The ratio of a chemical species’ formula weight, FW, to the number of its equivalents, n, is known as equivalent weight.

EW=FW/n

Relation between molarity and normality

Let us now take a look at the relation between molarity and normality. The concentration of a solution may be expressed in two ways: molarity and normality. The number of moles of solute per litre of solution is referred to as molarity, whereas the number of equivalents of solute per litre of solution is referred to as normality. The equivalency ratio, which is the number of equivalents of solute per mole of solute, connects the two expressions.

Below is the equation that shows the relation between molarity and normality.

Normality= n × Molarity

Where n = numbers of H+ in acid, OH– in base, and charge in ionic forms in salt.

When to use normality?

There are times when using normality rather than molarity, or another unit of concentration of a chemical solution is appropriate.

• In acid-base chemistry, normality refers to the concentration of hydronium (H3O+) and hydroxide (OH–). In this case, 1/feq is a positive integer.
• In precipitation processes, the equivalency factor, also known as normalcy, is used to calculate the number of ions that would precipitate. 1/feq is an integer number here as well.
• The equivalency factor in redox processes reveals how many electrons an oxidising or reducing substance can contribute or take. 1/feq can represent a fraction in redox processes.

Conclusion

In certain cases, normality isn’t the best unit of measurement. To begin, an equivalency factor must be defined. Second, the normality of a chemical solution is not a fixed value. Its value varies depending on the chemical process under investigation. A solution of CaCl2 that is 2N in terms of the chloride (Cl–) ion would only be 1N in terms of the magnesium (Mg2+) ion. This unit is discouraged by the International Union of Pure and Applied Chemistry, although it may be used in chemistry classrooms or in the lab, especially in acid-base titrations and redox reactions.