Solved problems in Conductometry

Conduction in Metals and in Electrolyte Solutions
Metallic Conductors
Electrolytic Conductors
Conduction in Electrolyte Solutions
Strong and Weak Electrolytes
Strong Electrolytes
Weak Electrolytes
Specific Conductance and Molar Conductance
Measurement of Molar Conductance
Determination of Cell Constant
Variation of Molar and Specific Conductance with Dilution
Kohlrausch’s Law of Independent Migration of Ions
Migration of Ions
Arrhenius Theory of Electrolytic Dissociation
Ostwald’s Dilution Law
Applications of Ostwald’s Dilution Law
Debye-Huckel-Onsagar Equation
Transport Numbers
Applications of Conductivity Measurements
1. Conduction in Metals and in Electrolyte Solutions
Conductors can be divided broadly into two categories:
(i) Metallic or electronic conductors
(ii) Electrolytic conductors
(i) Metallic Conductors
Metals are the best conductor and it remains unchanged with the passage of current. A metallic
conductor behaves as if it contains electrons which are relatively free to move. So electrons are
considered as charge carrier in metals. Therefore, these conductors are also called electronic
conductors. Metallic conduction or electronic conduction is the property possessed by pure metals,
most alloys, carbon and certain solid salts and oxides.
(ii) Electrolytic Conductors
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Conductors, through which passage of an electric current through them results in actual transfer
of matter or brings about a chemical change in them, are called electrolytic conductors or electrolytes.
Electrolytic conductors are of two types: -
(a) In the first category are electrolytic conductors, which conduct electrolytically in the
pure state, such as acids, bases and salt in water. e.g. NaCl, NaNO3, K2SO4 etc.
(b) In second category are generally put electrolytic conductors which consists of solutions
of one or more substances. Electrochemistry is mainly concerned with this type of
electrolytic conductor.
Generally electrolytic solutions are prepared by dissolving a salt, acid or base in water or other
solvents. There is a special class of conductors, which conduct partly electronically and partly
electrolytically, are known as mixed conductors. For example, solution of the alkali and alkaline earth
metals in liquid ammonia are mixed conductors. Fused cuprous sulphide conducts electronically, but a
mixture with sodium or ferrous sulphide also shows electrolytic conduction.
1.1 Conduction in Electrolyte Solutions
The passage of current through solutions of salts of metals such as zinc, iron, nickel, cadmium,
lead, copper, silver and mercury results in the liberation of these metals at the cathode and from
solutions of salts of the metals. If the anode consists of an attackable metal, the flow of the current is
accompanied by the passage of the metal into solution. When the anode is made of an inert metal, e.g.,
platinum, an element is generally set free at this electrode; from solutions of nitrates, sulphates,
phosphates, etc., oxygen gas is liberated, whereas from halide solutions, other than fluorides, the free
halogen is produced. The decomposition of solutions by the electric current, resulting in the liberation
of gases or metals, is known as electrolysis.
1.2 Strong and Weak Electrolytes
Solutes giving conducting solution in a suitable solvent are called electrolytes. On the basis of
degree of ionization, these electrolytes have been divided into two categories.
(i) Strong electrolytes
(ii) Weak electrolytes
1.2.1 Strong Electrolytes
Substances, which are highly dissociated and give solutions with high conductance in water, are
called strong electrolytes. Due to the high degree of dissociation of strong electrolytes these substances
are good conductor of electricity i.e., aqueous solutions of these substances have high value of molar
conductance and on dilution the increase in their molar conductance is very small. This is due to the
fact that such electrolytes are completely ionized at all dilutions therefore on further dilution the
number of current carrying particles does not increase in the solution. Thus, solutions of electrolytes
that have high molar conductance, and increases very slowly on dilution has a high degree of
dissociation is called strong electrolyte.
During the passage of an electric current through solutions, flow of electricity is associated with
the movement of particles, which are called ions. The ions carrying positive charges and moving in the
direction of the current, i.e., towards the cathode, are referred to as cations and those carrying a
negative charge and moving in the opposite direction, i.e., towards the anode, are called anions.
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1.2.2 Weak Electrolytes
Weak acids and weak bases, e.g., amines, phenols, most carboxylic acids and some inorganic
acids and bases, such as hydrocyanic acid and ammonia, and a few salts, e.g., mercuric chloride and
cyanide, are dissociated only to a small extent at reasonable concentration; this group of compounds in
general are called as weak electrolytes.
The molar conductance of the solutions of these electrolytes increases rapidly on dilution.
The reason of this is that more molecules ionize on dilution inspite of this they are never completely
ionized. For these electrolytes, the nature of the solvent is also important; a particular compound may
be strong electrolyte, being dissociated to large extent, in one solvent, but may behave as weak
electrolyte in other solvent due to low degree of dissociation.