Download Molar conductance

Electrical Resistance and Electrical Conductance:
Ohm’s law gives exact relationship between the resistance,
the current and voltage.
"If to the ends of a conductor a voltage "E" is applied and a
current "I" flows through it, then the resistance "R" of the
conductor:
(written Ω)
All electrolytic solution like metallic conductors obey
ohm’s law.
Conductance: Is the property of the conductor "metallic as
well as electrolytic" which facilitates the flow of electricity
through it.
conductance: the reciprocal of the electrical resistance is
called the conductance (c), thus:
(Ω-1), or mol or, Siemens (s).
Specific resistance & specific conductance:
The resistance (R) of a conductor is:
1- Directly proportional to its length (l) and,
2- Inversely proportional to its area of cross-section (a)
Specific resistance :
r : is a constant of proportionality, called specific resistance or
(resistivity). Its value depends upon the material of the
conductor.
Specific conductance:
"the reciprocal of specific resistance is known as specific
conductance or conductivity. It is denoted by ( ).
Thus, if
is the specific conductance and C is the
conductance of the solution, then:
specific conductance ( ): is defined as the conductance of a
solution of definite dilution. Enclosed in a cell having two
electrodes of unit area separated by 1cm length and 1cm2 as the
area of cross-section as shown in Fig.
isthe conductance of 1cm3 of the solution of the electrolyte.
Cell constant:
Cell constant =
Where (l) is the distance between the electrodes of the cell, and
(a) is the surface area of the electrode.
Thus cell constant may be obtained by measuring l & a.
Alternatively, this may be obtained as follows with the help of
the following equation.
Equivalent conductance and Molar conductance :
Equivalent conductance: Is the conductance of all ions
produced from 1g.eq. of the electrolyte dissolved in Vcm3 of the
solution when the distance between the electrodes is so large
that whole of the solution is contained between them.
It is represented by eq.
The equivalent conductance eq of a solution is calculated
from the specific conductance
. The relationship between Ʌ &
maybe obtained as follows:
Consider a rectangular vessel with its two opposite walls
1cm a part and made of some metal sheets so that they act as the
electrodes.
Case I :
Suppose 1cm3 of the solution containing 1g.eq. of the
electrolyte is taken in the vessel
The conductance c of this solution will be its specific
conductance
Further 1cm3 of the solution taken contains 1g.eq. of the
electrolyte, the conductance c of the solution will be its
equivalent conductance eq. (by definition) → c =eq.
Thus , when 1cm3 of the solution containing 1g.eq. of the
electrolyte is considered : eq=
Case II :
Suppose 4cm3 of the solution containing 1g.eq. of the
electrolyte is taken.
The conductance C of the solution will be still equal to its
equivalent conductance eq at this dilution, but now there will
be 4 cubes each of volume 1cm3 as shown in Fig.
For each 1cm3 of the solution
So that the total conductance C of the solution, i.e.,
equivalent conductance eq is 4 times the specific conductance
.
equivalent conductance = specific conductance * V
eq
Where V is the volume in cm3 containing 1g.eq. of the
electrolyte
In terms of concentration, if the solution has a concentration of
C g.eq. /l, i.e., c g.eq’s. are present in 1000 cm3 of the solution
then the volume of the solution containing 1g.eq. will be :
eq
Molar conductance :
Is the conductance of all the ions produced from 1mole of
the electrolyte dissolved in V cm3 of the solution when the
electrodes are 1cm apart and the area of the electrodes is so
large that the whole of the solution is contained between them. It
is usually represented by m
Molar conductance m is related to the specific
conductance as follows :
Molar conductance = specific conductance * volume in cm3
containing 1mol of the electrolyte
m
m = ohm-1 . cm2 . mol-1
= S cm2 mol-1
= Ω-1 cm2 mol-1
or
or