Determination of equivalent masses of elements

Determination of equivalent masses of elements

Equivalent masses can be determined by the following methods:

1.     Hydrogen displacement method

2.     Oxide method

3.     Chloride method

4.     Metal displacement method

1.Hydrogen displacement method

This method is used to determine the equivalent mass of those metals such as magnesium, zinc and aluminium which react with dilute acids and readily displace hydrogen.

Mg + 2HCl -> MgCl2 + H2

Zn + H₂SO_{4 - >} ZnSO₄ + H₂

2Al + 6HCl - > 2AlCl₃ + 3H₂

From the mass of the metal and the volume of hydrogen displaced, the equivalent mass of the metal can be calculated.

2.Oxide Method

This method is employed to determine the equivalent mass of those elements which readily form their oxides eg. magnesium, copper etc. Oxide of an element can be formed by direct method or by indirect method.

Magnesium forms its oxide directly on heating

2Mg + O2  ® 2 MgO

In the case of copper, its oxide is obtained in an indirect manner i.e. copper is dissolved in concentrated nitric acid and the copper(II) nitrate formed after evaporation is strongly heated to give copper (II) oxide.

Cu + 4HNO3 ® Cu(NO3)2  + 2H2O + 2 NO2

2Cu(NO3)2  ® 2CuO + 4 NO2 + O2

Calculations

Mass of the element taken = w1 g

Mass of the oxide of the element = w₂ g

Mass of oxygen = (w₂-w₁) g

(w₂ - w₁) g of oxygen has combined with w₁ g of the metal.

8 g of oxygen will combine with w1 / (w2 - w1) x 8

This value represents the equivalent mass of the metal.

3.Chloride Method

The equivalent mass of those elements which readily form their chlorides can be determined by chloride method. For example, a known mass of pure silver is dissolved completely in dilute nitric acid. The resulting silver nitrate solution is treated with pure hydrochloric acid when silver chloride is precipitated. It is then filtered, dried and weighed. Thus from the masses of the metal and its chloride, the equivalent mass of the metal can be determined as follows :

4.Calculations

Mass of the metal = w1 g

Mass of the metal chloride = w2 g

Mass of chlorine = (w₂ - w₂) g

(w₂ - w₁) g of chlorine has combined with w₁ of the metal

35.46 g of chlorine will combine with 35.46 x w1 / (w2 - w1) g of the metal

This value gives the equivalent mass of the metal.

Uses of volumetric laws

If the volume of the acid is represented as V₁, the normality of the acid as N₁, volume of base as V₂ the normality of the base as N₂, then from the law of volumetric analysis it follows that

V1 x N1 = V2 x N2

All volumetric estimations are based on the above relationship. If any three quantities are known, the fourth one can readily be calculated using the above expression.