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Transcript
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.