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=
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Atomic weight
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Valency
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=
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A
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x
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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=
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Molecular weight of acid
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Basicity
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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.
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Phosphorous based acids
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 |
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Figure 1: Furnishable hydrogen
ions of acids
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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=
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Molecular weight of base
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Acidity
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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=
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Molecular weight of compound
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Numbers of electronic charge on
cation or anion
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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=
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Molecular weight of Radical A
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Numbers of electronic charge
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+
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Molecular weight of Radical B
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Numbers of electronic charge
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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=
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Molecular weight of ion
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Numbers of electronic charge
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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=
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Molecular weight of compound
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Numbers of electrons transferred in
redox reaction
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Alternatively,
Equivalent weight,E=
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Molecular weight of compound
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Change in oxidation number in redox
reaction
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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++6e−→2K++2Cr3++7H2O
Equivalent weight ofK2Cr2O7=
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294.2
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6
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=49
Study of redox
reaction is in itself an exclusive and extensive topic. We shall, therefore,
discuss redox reaction separately.
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