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How to calculate equivalent weight ?

Eequivalent weight Calculations

So, if you want to calculate equivalent weight you must have a clear concept on valency factor which we say n-factor , Equivalent weight of any molecule can be define as Molecular weight divided by n-factor , so let’s understand the meaning of n-factor to have clear idea of calculating the  equivalent weight of any molecules.

Equivalent weight. =  Molecular weight/n-factor

The equivalent of a substance is calculated as

    ∴    Number of equivalents ==

    ∴    Number of equivalents = no. of moles × n factor

   

n Factor Calculation

Since, equivalent of a substance = and eqv. mass =

Thus, to calculate the equivalents of any substance, the knowledge of n factor calculation is a must. The n factor of various substances is calculated as:

1.    Acids : The number of moles of replaceable H+ ions per mole of the acid i.e. basicity.

For example, HNO3; n = 1; H2SO4; n =2; H3PO4; n = 3, H3PO3; n = 2, H3BO3; n = 1

All these values are the maximum values of n factor for the corresponding acid e.g. the n factor for H2SO4 may be 1 or 2 depending on the reaction it undergoes.

H2SO4 + NaOH NaHSO4 + H2O        …(i)

H2SO4 + 2NaOH Na2SO4 + H2O        …(ii)

For bases: Bases will be treated as species which furnish OH− ions when dissolved in a solvent. The n-factor of a base is the no. of OH− ions that a molecule of the base would give when dissolved in a solvent (Acidity).

For example,  NaOH (n = 1), Ba(OH)2 (n = 2), Al(OH)3 (n = 3), etc.

For salts: A salt reacting such that no atom of the salt undergoes any change in oxidation state.

For example,   2AgNO3  + MgCl2   Mg(NO3)2 + 2AgCl

In this reaction, it can be seen that the oxidation state of Ag, N, O, Mg and Cl remains the same even in the product. The n-factor for such a salt is the total charge on cation or anion.

InRedox Change

For oxidizing agent or reducing agent n-factor is the change in oxidation number per mole of the substance.   

SOME OXIDIZING AGENTS /REDUCING AGENTS WITH Equivalent weight

Species

Changed to

Reaction

Electrons exchanged or change in O.N.

Eq. wt.

MnO4-(O.A.)

Mn2+
in acidic medium

MnO4- + 8H+ +5e- →Mn2++ 4H2O

5

MnO4-(O.A.)

MnO2
in neutral medium

MnO4- + 3e- +2H2O→ MnO2 + 4OH-

 

3

MnO4-(O.A.)

MnO4 2-
in basic medium

MnO4- + e- MnO42-

1

Cr2O72-(O.A.)

Cr3+
in acidic medium

Cr2O72- +14H++6e → 2Cr3+ + 7H2O

6

MnO2(O.A.)

Mn2+
in acidic medium

MnO2 + 4H++2e-→Mn2+ +2H2O

2

Cl2(O.A.)

(in bleaching powder)

Cl-Cl-

Cl2 + 2e- →2Cl-

2

 

Species

Changed to

Reaction

Electrons exchanged or change in O.N.

Eq. wt.

 CuSO4(O.A.)

(in iodometric titration)

Cu+

Cu2+ +e- Cu+

 

1

S2O32-(R.A.)

S4O62-

2S2O32- → S4O62- +2e-

2 (for two molecules)

H2O2(O.A.)

H2O

H2O2 +2H+ +2e-→2H2O

2

H2O2(R.A.)

O2

H2O2 O2 +2H+ + 2e-(O. N. of oxygen in H2O2 is −1 per atom)

2

Fe2+(R.A.)

Fe3+

Fe2+ Fe3+ +e-

1

 

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