Equivalent weight

 Equivalent weight

Definition

Equivalent weight is expressed as the ratio of atomic weight or molecular weight or formula weight (or molar mass) to the valency.

Equivalent weight of a substance is the number of parts by weight or amount of a substance, which will combine with or displace 1 part by weight of hydrogen, or 8 parts by weight of oxygen or 35.5 parts by weight of chlorine. It is expressed in amu.

In simple words, equivalent weight of a species is its combining capacity with other standard species under given conditions.

If the element reacts directly with H₂ or O₂ or Cl₂ then equivalent of element is calculated as (both masses are in same unit):

Formula

Equivalent weight of different substances is given by:

Equivalent weight of element = Atomic weight / valency

Equivalent weight of acid   = Molecular weight/ basicity (ionizable H⁺ ions)

Equivalent weight of base    = Molecular weight/ acidity (ionizable OH⁻ ions)

Equivalent weight of salt     = Molecular weight/ no of positive charge on cation

Equivalent weight of oxidant = Molecular weight/ no. of electrons gained (change in oxidation no.)

Equivalent weight of reductant   = Molecular weight/ no. of electrons lost(change in oxidation no.)

Equivalent weight of radical = Formula weight/valency

Gram Equivalent

Equivalent weight of a substance expressed in gram is called Gram equivalent weight or One gram equivalent.

For example;

One gram equivalent of Na = 23 g

One gram equivalent of Ca = 20 g

One gram equivalent of Al = 9 g

One gram equivalent of HCl = 36.5 g

One gram equivalent of H₂SO₄= 49 g

Equivalent weight of CO₃²⁻ = 12+48/2 = 60/2= 30

Equivalent weight of KMnO₄ in acidic medium = 39+55+64/5 = 158/5= 31.6

Equivalent weight of Acids

Equivalent weight of an acid is the number showing how many parts by weight of an acid contains one part by weight of replaceable hydrogen i.e.

For example

1. Equivalent weight of HCl    = 36.5/1= 36.5

2. Equivalent weight of HNO₃ = 63/1   = 63

3. Equivalent weight of CH₃COOH = 60/1   = 60

4. Equivalent weight of H₂SO₄ = 98/2   = 49

5. Equivalent weight of H₂C₂O₄.2H₂O = 126/2 =63

Equivalent weight of an acid is the number showing how many parts by weight of an acid neutralizes one gram equivalent an alkali (base)

For example

equivalent weight of H₂SO₄ can be calculated as follows:

Equivalent weight of Base

Equivalent weight of a base is the number showing how many parts by weight of a base contains one part by weight of replaceable OH⁻ ion i.e.

For example

1 Equivalent weight of NaOH    = 40/1    = 40

2 Equivalent weight of KOH       = 56/1   = 56

3 Equivalent weight of NH₄OH  = 35/1   = 35

4 Equivalent weight of Ca(OH)₂ = 74/2  = 37

Equivalent weight of a base is the number showing how many parts by weight of an alkali neutralizes one gram equivalent an acid.

For example

Equivalent weight of NaOH can be calculated as follows:

Equivalent weight of Compounds (salts)

Equivalent weight of a compound (salt) is the number showing how many parts by weight of a compound (salt) contains one, two or three positive charge on its cation. i.e.

For example

1. Equivalent weight of Na₂CO₃ = 106/2 = 53 amu

2. Equivalent weight of K₂CO₃    = 138/2  = 69 amu

3. Equivalent weight of CaCO₃    = 100/2= 50 amu

4. Equivalent weight of NaHCO₃ =  84/1 = 84 amu

2.         Equivalent weight of a compound (salt) is the number showing how many parts by weight of a compound (salt) interacts with the known chemical equivalent an element or a compound (whose equivalent weight is known).

For example

Equivalent weight of Na₂CO₃ can be calculated as follows:          

Equivalent weight of Oxidizing agent

The equivalent weight of an oxidizing agent or oxidant is that weight of it which gains one electron. It is obtained by dividing molecular weight by no of electrons gained per mole.

For example

1. Equivalent of an oxidant KMnO₄ in acidic medium can be calculated as:

2KMnO₄ + 3H₂SO₄ →K₂SO₄ + 2MnSO₄ + 3H₂O + 5[O]

 Equivalent weight of KMnO₄  in acidic medium =  158/5  =  31.6

2.Equivalent of an oxidant KMnO₄ in basic medium can be calculated as:

2KMnO₄ + H₂O→ 2KOH + 2MnO₂ +  + 3[O]

Equivalent weight of KMnO₄  in basic medium =  158/3  =  52.66

Equivalent weight of reducing agent

The equivalent weight of a reducing agent or reductant is that weight of it which donates one electron. It is obtained by dividing molecular weight by no of electrons lost per mole.

For example

1).        Equivalent of a reducing agent FeSO₄.7H₂O can be calculated as:

             Equivalent weight of FeSO₄.7H₂O =  278/1  =  278

2).        Equivalent of a reducing agent Mohr’s salt (FeSO₄. (NH₄)₂SO_4.6H₂O can be calculated as:

             Equivalent weight of FeSO₄.7H₂O =  392/1  =  392