12. Equivalent Weight

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Equivalent weight of Element:
Equivalent weight of an element is defined as the number of parts by weight of that element which can combine or displace directly or indirectly 1.008 parts by weight of hydrogen or 8 parts by weight of oxygen or 35.5 parts by weight of chlorine or their equivalent.
Mathematically, 





i.e., E= A/V


Gram Equivalent Weight:
When Equivalent weight of substance is expressed in gram, then it is called gram equivalent weight. 








2×1.008 parts by wt. of hydrogen is displaced by 24 parts by weight of mg.
1.008 parts by wt. of hydrogen is displaced by 24/2 = 12 parts by wt. of mg.
12gm of mg = 1gram equivalent weight of mg.

Q. Calculate the number of gram equivalent present in 60gm Hg.
Solution, 
Wt. of mg = 60gm, Eq. Wt. of mg = 12
No. of gram equivalent = 60/12 = 5

Variable Equivalent Weight:-
Equivalent weight of an element is the ratio of atomic weight with valency. Atomic weight of an element is fixed but valency of some element is variable. So equivalent weight of the such elements whose valency is variable can also be variable. 
For Example:-












Relation Between Atomic Weight (A), Equivalent Wt. (E) and Valency (V).
Valency is the number of hydrogen atoms with which an atom of element can combine. Let us consider an element having atomic weight (A), Equivalent Weight (E) and valency (V), Now,
V atoms of hydrogen can combine with 1 atom of that element.
V×1.008 parts by wt. of hydrogen can combine with 1 atom of that element.
1.008 parts by wt. of hydrogen can combine with A parts by wt. of that element.
E parts by wt. of hydrogen can combine with A/V parts by wt. of that element.








Equivalent weight of Acid:-
Equivalent weight of acid is defined as the number of parts by weight of that acid which contains one replaceable hydrogen, for example:-
In HCl
36.5 parts by wt. of HCl contains one replaceable hydrogen
So, equivalent weight of HCl is 36.5.
Similarly in H2SO4.
98 parts by wt. of H2SO4 contains 2 replaceable hydrogen.
98/2 parts by wt. of H2SO4 contains 1 replaceable hydrogen.
ie, Equivalent wt. of H2SO4 = 49





Basicity is the number of replaceable of hydrogen present in a molecule of an acid.
Depending on the number of replaceable hydrogen present in a molecule, acids are classified as monobasic acid, dibasic acid, tribasic acid etc.
Monobasic Acid
Replaceable Hydrogen
Hydrochloric Acid – HCl
Nitric Acid – HNO3
Acetic Acid – CH3COOH
1
1
1
Dibasic Acid
Replaceable Hydrogen
Sulphuric Acid – H2SO4
Oxalic Acid – COOH
                       |COOH
2
2
Tribasic Acid
Replaceable Hydrogen
Phosphoric Acid – H3PO4
3

Equivalent Weight of Base:
Equivalent weight of a base is defined as the number of parts by weight of that base which can completely neutralise one gram equivalent of an acid.
Mathematically, 





Acidity is the number of hydroxyl group present in a molecule of a base for oxides. It is twice the number of oxygen.

For Example,









Equivalent Weight of Salt:
Equivalent weight of salt is the ratio of molecular weight of salt with total positive charges present on cations.









Determination of Equivalent Weight of Metal:
a] Oxide Formation Method:
In this method a small piece of pure metal is weighed exactly. This metal piece is converted into its oxide directly or indirectly. After this, this metal oxide is also weighed then weight of oxygen consumed during the formation of oxide is calculated. Finally equivalent weight of metal is calculated as:-






Observation and Calculation:
Weight of crucible = w1 gm
Weight of metal + crucible = w2 gm
Weight of metal oxide + crucible = w3gm 
Weight of metal = (w2-w1) gm
Weight of oxygen = (w3-w2) gm






                                        = E

ii] Hydrogen Displacement Method
This method is applicable for only those metals which can displace hydrogen from dilute acid. In this method a small piece of pure metal is weighed correctly and allowed to react with dilute acid. The volume of hydrogen gas is collected. This volume is converted into STP by using combined gas equation. After this weight of evolved hydrogen is calculated finally.


















Observation and Calculation:
Weight of metal piece = W1 gm
Volume of hydrogen displaced = V1
Pressure of hydrogen (P1) = p-f
Here, p is moist hydrogen pressure,
          f is aqueous tension,
Temperature = T1

At STP,
Pressure (P2) = 760mmHg
Temperature (T2) = 273K
Volume (V2) = ?





 

















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