Equivalent weight is derived from the molecular masses of compounds. Molar mass and n- factor of the substance play an important role in determining equivalent weight of  substance.

The mass of a substance which displaces or combines (by weight) a certain amount of other substance is called equivalent weight. Equivalent weight is derived from the molecular masses of compounds. Before understanding the methods to determine equivalent weight, there are few chemical terms which should be studied in order to understand the methods of determination of molecular weight.

Mole

Ostwald coined the term mole. One mole of a substance is the amount of which contains as many atoms, molecules or ions as present in 12 grams of carbon-12.

1 mol= 6.023× 10²³  atoms = one gram atom = gram atomic mass

1 mol= 6.023× 10²³ molecules = gram molecular mass

In gaseous state, at Standard Temperature and Pressure (T = 273 K, P = 1 atm)

Gram molecular mass = 1 mol = 22.4 L = 6.023× 10²³  molecules

6.023× 10²³ is known as AVOGADRO NUMBER and is denoted by NA.

Number of moles = amount of a substance (in gram) / molar mass

Number of molecules = number of moles × NA

Atomic Mass

Atomic mass is the mass of a single atom. Its unit is amu (atomic mass unit). One atomic mass unit (amu) is the mass of a substance which is exactly equal to 1/12th the mass of a carbon-12 atom. It is denoted as a subscript of an atomic symbol. For example, the atomic mass of

14N7   =  atomic mass of nitrogen atom is 14 amu.

Molecular Mass

Molecular mass is the mass of a molecule, i.e. the number of times a molecule is heavier than 1/12th  the mass of a carbon-12 atom. Its unit is Daltons. Molecular mass of a substance is an additive property and can be calculated by taking the algebraic sum of atomic masses of all the atoms of different elements present in one molecule. For example, the molecular mass of water can be calculated as-:

H2O = 2 Hydrogen atoms + 1 Oxygen atom

= 2 ✖  1 + 1 ✖  16

= 2 + 16

= 18

Formula Mass

There are some substances which do not contain individual molecules as their constituent units,  such as sodium chloride. For such types of compounds, formula mass is calculated instead of molecular mass as sodium chloride does not exist as a single entity in solid state.

For example, the formula mass of NaCl = mass of Sodium + mass of Chlorine

= 23 + 35.5 u

= 58.5 u

Equivalent Mass

The mass of a substance which displaces or combines (by weight) a certain amount of other substance is called equivalent weight. Generally, equivalent mass is defined as the mass of element or a compound which combines or displaces 1 part of Hydrogen or 8 parts of Oxygen or 35.5 parts of Chlorine.Equivalent weight is also known as GRAM EQUIVALENT as it is the mass of one equivalent.

Equivalent weight of  METAL = (weight of a metal /weight of Hydrogen displaced) ✖ 1.008

Or                                           = (weight of metal /weight of oxygen combined) ✖ 8

Or                                            = (weight of metal /weight of chlorine combined) ✖ 35.5

Or                                            = (weight of metal /volume of Hydrogen displaced at STP) ✖ 11200

In general,

Weight of substance A/ weight of substance B = Equivalent weight of A/ Equivalent weight of B

If a compound (I) is being converted into another compound (II) of the same metal,

Weight of compound (I) = Equivalent weight of metal + Equivalent weight of anion of compound (I)

Weight of compound (II)=Equivalent weight of metal + Equivalent weight of anion of compound (II)

What is the need of Equivalent Weight?

Equivalent weight is very useful in analytical chemistry. While performing certain experiments, the value of equivalent weight plays a major role. Some of the experimental techniques which requires the value of equivalent weight are listed below-:

• Volumetric Analysis – While performing titrations, primary standards are used which are standardised solutions ( solution of known strength) . While preparing the primary standard, it is very important to know the equivalent weight of the compound. Equivalent weight is also used to determine the Normality of a solution.

Normality of a solution = Equivalent weight or gram equivalent / volume of solvent

• Gravimetric Analysis – In gravimetric analysis of metals, the mass of the precipitate which corresponds to one gram of metal is called equivalent weight.
•  Polymer Chemistry – In polymer chemistry, the mass of a reactive polymer which has one equivalent of reactivity is called equivalent weight of a reactive polymer.

Determination of Equivalent Weight of Substances

Equivalent weight of a substance can be determined by theoretical methods. The methods used to determine equivalent weight are listed below-

• THEORETICAL METHODS – For determining the equivalent weight of a substance by theoretical method, molar mass of the substance plays an important role. Another tool is n-factor. Calculation of equivalent weight by using n-factor can be performed as,

Equivalent weight = Atomic mass or molecular mass / n-factor

n-factor for various compounds can be obtained as-

• FOR ACIDS – In the case of acids, the number of ionisable H+ per molecule is the value of n-factor. It is called the basicity of the acid.

• FOR BASES – In case of bases, the number of ionisable    per molecule is the value of n- factor. It is called acidity of the base.

• FOR IONS – In case of ions, n-factor is equal to the charge of that ion.
• FOR REDOX TITRATIONS – In case of redox titrations, n-factor is equal to change in oxidation number .

Conclusion

One mole of a substance is the amount of which contains as many number of atoms, molecules or ions as present in 12 grams of carbon-12 .The mass of a substance which displaces or combines (by weight) a certain amount of other substance is called equivalent weight.Equivalent weight is very useful in analytical chemistry. While performing certain experiments, the value of equivalent weight plays a major role. Molar mass and n- factor of the substance plays an important role in determining equivalent weight of  substance.