Chemical Eequivalence

Significance of Equivalent WeightAn equivalent of a substance is defined as the amount o it which combines with 1 mole of hydrogen atoms or replaces the same number of hydrogen atoms in a chemical reaction. The weight in grams of 1 equivalent is called the equivalent weight in grams. For example, in the compounds HBr one mole of H combines with one mole of Br, 1/2 mole of O and 1/3 mole of N respectively. Hence the equivalent weights in grams of Br, O and N are the weights of 1 mole each of Br, O and N contains their 1 equivalent, 2 equivalents and 3 equivalents respectively.Thus : eq.wt. of Br= 1x79.9=79.9
To determine the equivalent weight of an element, it is not necessary to proceed from its hydrogen compound only. Eequivalent weight of an element can be calculated using the composition of the compound of the given element with any other element, whose equivalent weight is known by the knowledge of the Law of Equivalence. The law states that one equivalent of an element combines with one equivalent of the other. Accordingly, the equivalent weight of an element is the weight of its moloe combining with one equivalent of another element.The equivalent weight of a compound taking part in a reaction is the weight of the compound which combines with 1 equivalent of another compound. Thus knowing that 1 mole of HCl is equal to 1 equivalent of it, equivalent weight of Sodium carbonate can be calculated from the the equation. In an acid-base neutralisation reaction the equivalent weight of an acid is that portion of the weight of 1 mole of the acid which can furnish 1 mole of H, and the equivalent weight of a base is the portion of weight of one mole of the base which can furnish 1 mole of OH or accept 1 mole of H. In redox reactions, the equivalent weight of an oxidising or a reducing agent is the portion of the weight of 1 mole of substance that picks up or releases 1 mole of electrons respectively.