Equivalent mass of acid, base, salt, oxidising agent and reducing agent

Equivalent mass of acid, base, salt, oxidising agent and reducing agent

Acids contain one or more replaceble hydrogen atoms. The number of replaceble hydrogen atoms present in a molecule of the acid is referred to its basicity.

1.Equivalent mass of the acid

Equivalent mass of an acid is the number of parts by mass of the acid which contains 1.008 part by mass of replaceble hydrogen atom.

Equivalent mass of the acid  =        molar mass of an acid / No. of replaceble hydrogen atom

 = molar mass of the acid / basicity of the acid

For example, the basicity of sulphuric acid is 2.

Equivalent mass of H2SO4 = Molar mass of H2SO4 / 2  = 98/2 = 49

2.Equivalent mass of the base

Equivalent mass of a base is the number of parts by mass of the base

which contains one replaceable hydroxyl ion or which completely neutralises one gram equivalent of an acid. The number of hydroxyl ions present in one mole of a base is known as the acidity of the base. Sodium hydroxide, potassium hydroxide, ammonium hydroxide are examples of monoacidic bases.

Calcium hydroxide is a diacidic base

In general,           

equivalent mass of a base      = molar mass of the base / acidity of the base

Equivalent mass of KOH = 56 /1 = 56

3.Equivalent mass of a salt

Equivalent mass of a salt is a number of parts by mass of the salt that is produced by the neutralisation of one equivalent of an acid by a base.

In the case of salt like potassium chloride, the salt formed by the neutralisation of one equivalent of an acid by a base.

KOH + HCl -> KCl + H2O

Therefore, the equivalent mass of the salt is equal to its molar mass.

4.  Equivalent mass of an oxidising agent

The equivalent mass of an oxidising agent is the number of parts by mass which can furnish 8 parts by mass of oxygen for oxidation either directly or indirectly.

For example, potassium permanganate is an oxidising agent. In acid medium potassium permanganate reacts as follows

2 KMnO4 + 3 H2SO4 ->  K2SO4 + 2 MnSO4 + 3 H2O + 5 [O] 316 80

80 parts by mass of oxygen are made available from 316 parts by mass of KMnO4

8 parts by mass of oxygen will be furnished by

(316 x 8 ) / 80 = 3.16

Equivalent mass of KMnO₄  = 31.6g equiv^-1

5. Equivalent mass of a reducing agent

The equivalent mass of a reducing agent is the number of parts by mass of the reducing agent which is completely oxidised by 8 parts by mass of oxygen or with one equivalent of any oxidising agent.

(i) Ferrous sulphate reacts with an oxidising agent in acid medium according to the equation

2 FeSO₄ + H₂SO₄ + (O) ® Fe₂ (SO₄)₃ + H₂O

2 x 152g  16g

16 parts by mass of oxygen oxidised 304 parts by mass of ferrous sulphate

8 parts by mass of oxygen will oxidise 304/16 x 8 parts by mass of ferrous sulphate.

The equivalent mass of crystalline ferrous sulphate, FeSO4 7H₂O is 152 + 126 = 278

126 is the mass corresponding to 7 moles of water.

(ii) In acid medium, oxalic acid is oxidised according to the equation

(COOH)2 + (O) ® 2 CO2 + H2O

16 Parts by mass of oxygen oxidised 90 parts by mass of anhydrous oxalic acid.

8 parts by mass of oxygen will oxidise 90/16 x 8 = 45 parts by mass of anhydrous oxalic acid.

 Equivalent mass of anhydrous oxalic acid = 45 g equiv^-1

But equivalent mass of crystalline oxalic acid, (COOH)2. 2H2O=126/2 = 63 g equiv^-1.