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Equivalent weight •Equivalent weight (also known as gram equivalent) is a term which has been used in several contexts in chemistry. •In its most general usage, it is the mass of one equivalent, that is the mass of a given substance which will: •supply or react with one mole of hydrogen ions(H+) in an acid–base reaction; or •supply or react with one mole of electrons in a redox reaction. •Equivalent weight has the dimensions and units of mass, unlike atomic weight, which is dimensionless. •Equivalent weights were originally determined by experiment, but (insofar as they are still used) are now derived from molar masses. Equivalent Mass and Number of Electrons Equivalent mass is equal to the molecular or atomic mass divided by the number of electrons involved in the reaction per molecule, atom or ion. For example in the reaction, two electrons are needed to produce one molecule of hydrogen gas. So, 2 Faraday of electricity is needed to produce one mole of hydrogen gas. Hence, Equivalent Mass and Number of Electrons For the reaction, per mole of copper atoms, 2 faradays are needed. So, the equivalent mass of copper is half of its atomic mass. The equivalent mass of any species is not simply a property of the species, but depends upon the reaction in which it participates, i.e., one chemical species can have more than one value for its equivalent mass depending upon the reaction it participates. The equivalent mass of a substance is the quantity of material deposited or dissolved by 1 F (= 96500 C) of electricity. Normality Normality: There is a relationship between normality and molarity. Normality can only be calculated when we deal with reactions, because normality is a function of equivalents. The example below uses potassium hydroxide (KOH) to neutralize arsenic acid. By studying the reaction it is possible to determine the proton exchange number to determine the normality of the arsenic acid. Look at the equation H3AsO4 + 2KOH --> K2HAsO4 + 2H2O: Equivalent weight = molar mass/(H+ per mole) Equivalent = mass of compound / Equivalent weight And Normality = (equivalents of X)/Liter And the part that is of interest to you is that Normality = molarity x n (where n = the number of protons exchanged in a reaction). Normality You probably remember that when a hydrogen atom is ionized and loses its electron, you are left with only a proton. So a hydrogen ion is basically a proton. Let's assume that we have a 0.25 M solution of H3AsO4 and want to determine the normality of it if it participates in the reaction H3AsO4 + 2KOH --> K2HAsO4 + 2H2O When H3AsO4 is neutralized by KOH, H3AsO4 provides two protons to form 2H2O. Note that H3AsO4 has three hydrogens, but K2HAsO4 only has one hydrogen. That means that 2 protons were exchanged, Again normality = molarity * n Normality Remember that normality of the solution is 0.25 mol H3AsO4 and there were two protons exchanged (2 equivalents/mole) So, in short, while there is a relationship between the normality of a solution and the molarity of a solution, the normality can only be determined by examining reaction, determining the proton exchange and multiplying molarity by that number.