Equivalent weight of elements, and compounds

Equivalent weight of an element

In the case of an element, the equivalent weight is defined as :

Equivalent weight,E=

Atomic weight
Valency
=
A
x

Note that atomic weight substitutes molecular weight and valency substitutes valence factor in the definition. Valencies of hydrogen, calcium and oxygen are 1,2 and 2 respectively. Hence, their equivalent weights are 1/1 =1, 40/2 = 20 and 16/2 = 8 respectively.

Equivalent weight of an acid

The valence factor of an acid is equal to its basicity. The basicity of an acid is equal to furnishable hydrogen ion (proton) in its aqueous solution. Importantly, basicity is not same as the number of hydrogen atoms in acid molecule. Consider acetic acid (CH3COOH). It contains 4 hydrogen atoms in it, but only 1 furnishable hydrogen ion. As such, basicity of acetic acid is 1. With this background, we define equivalent weight of an acid as :

Equivalent weight,E=

Molecular weight of acid
Basicity

Basicity of sulphuric acid is 2. Hence, equivalent weight of sulphuric acid (H2SO4) is (2X1 + 32 + 4X16)/2 = 98/2 = 49. Similarly, basicity of oxalic acid is 2. Hence, equivalent weight of oxalic acid (H2C2O4 ) is (2X1 + 2X12 + 4X16)/2 = 90/2=45.

Phosphorous based acids like phosphoric acid (H3PO4 ), phosphorous acid (H3PO3) and hypo-phosphorous acid (H3PO2) need special mention here to understand their basicity. The structures of three acids are shown here. From the structure, it appears that these compounds may furnish OH ions, but bond strengths between phosphorous and oxygen (P-O) and phosphorous and hydrogen (P-H) are stronger than between oxygen and hydrogen (O-H) in –OH group. As such, these molecules release hydrogen ions from –OH group and behave as acid. Clearly, basicities of phosphoric acid (H3PO4), phosphorous acid (H3PO3) and hypo-phosphorous acid (H3PO2) are 3, 2 and 1 respectively.

Figure 1: Furnishable hydrogen ions of acids
Phosphorous based acids
 Phosphorous based acids  (ge1.gif)

Equivalent weight of a base

The valence factor of a base is equal to its acidity. The acidity of a base is equal to furnishable hydroxyl ion (OH-) in its aqueous solution. With this background, we define equivalent weight of a base as :

Equivalent weight,E=

Molecular weight of base
Acidity

Acidity of KOH is 1, whereas acidity of Ca(OH)2 is 2. Hence, equivalent weight of KOH is (39 + 16 + 1)/1 = 56/1 = 56. Similarly, equivalent weight of Ca(OH)2 is {40 + 2X(16+1)}/2 = 74/2=37.

Equivalent weight of a compound

The valence factor of a compound depends on the manner a compound is involved in a reaction. The compounds of alkali metal salts and alkaline earth metal salts are, however, constant. These compounds are ionic and they dissociate in ionic components in aqueous solution. In this case, valence factor is equal to numbers of electronic charge on either cation or anion.

Equivalent weight,E=

Molecular weight of compound
Numbers of electronic charge on cation or anion

The numbers of electronic charge on cation of NaHCO3 is 1. Hence, equivalent weight of NaHCO3 is (23 + 1 + 12 + 3X16)/1 = 84.

If we look at the defining ratio of equivalent weight of a compound (AB) formed of two radicals (say A and B), then we can rearrange the ratio as :

Equivalent weight, E=

Molecular weight of Radical A
Numbers of electronic charge
+
Molecular weight of Radical B
Numbers of electronic charge

Thus,

Equivalent weight of AB=Equivalent weight of A+Equivalent weight of B

Equivalent weight of an ion

The valence factor of an ion is equal to numbers of electronic charge on the ion. Therefore, we define equivalent weight of an ion as :

Equivalent weight,E=

Molecular weight of ion
Numbers of electronic charge

The numbers of electronic charge on carbonate ion (CO32 ) is 2. Hence, equivalent weight of carbonate ion is (12 + 3X16)/1 = 60/2 = 30. Similarly, equivalent weight of aluminum ion (Al3+) is 27/3 = 9.

Equivalent weight of an oxidizing or reducing agent

In a redox reaction, one of the reacting entities is oxidizing agent (OA). The other entity is reducing agent (RA). The oxidizer is recipient of electrons, whereas reducer is releaser of electrons. The valence factor for either an oxidizing or reducing agent is equal to the numbers of electrons transferred from one entity to another.

Equivalent weight,E=

Molecular weight of compound
Numbers of electrons transferred in redox reaction

Alternatively,

Equivalent weight,E=

Molecular weight of compound
Change in oxidation number in redox reaction

Potassium dichromate in acidic medium is a strong oxidizer. It means it gains electrons during redox reaction. Potassium dichromate in acidic solution results in :

K2Cr2O7+14H++6e2K++2Cr3++7H2O

Equivalent weight ofK2Cr2O7=

294.2
6
=49

Study of redox reaction is in itself an exclusive and extensive topic. We shall, therefore, discuss redox reaction separately.

6 comments:

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Unknown said...

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