Kd=1.74 X 10-5

1. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
Dr. Mithil Fal Desai
Aim: Determination of equivalence conductance, degree of dissociation and dissociation
constant of weak acid.
Chemicals: Acetic acid.
Apparatus: Beakers, standard volumetric flask and glass rod.
Theory: Conductance is the reciprocal of electrical resistance. Unit for conductance is mho or
siemens (S) [1 S = 1 ohm-1
]. Specific conductivity (K) is defined conductance of solution of 1 cm
length and having 1 cm2
area or conductance of 1 cm3
of the solution of electrolyte.
Specific conductance = Measured conductance (ohm-1
) X Cell constant (cm-1
) --- eq1
Equivalent conductance (ᴧeq) at dilution V cm3
is defined as the conductance of all ions
produced from one gram equivalent electrolyte dissolved in V cm3
when the distance between
the electrodes is one and electrodes are so large that the whole solution is contained between
them. Molar conductivity (ᴧm) of solution at dilution V cm3
, is the conductance of all the ions from
one mole of electrolyte dissolved in V cm3
when the distance between the electrodes is one and
electrodes are so large that the whole solution is contained between them.
ᴧm = K X 1000/Molarity --- eq2
ᴧeq= K X 1000/Normality --- eq3
Degree of dissociation (α) is the ratio of equivalent conductance (ᴧeq) to the conductance at
infinite dilution (ᴧ∞).
(α)= (ᴧeq)/ (ᴧ∞) --- eq4
ᴧ∞ = VA ᴧ∞
+
+VB ᴧ∞
-
(Kohlrausch’s law) --- eq5
Where VA and VB is the number of ions dissociated from molecule cation and anion respectively.
VA and VB is a unity for acetic acid. ᴧ∞
+
(H+
) and ᴧ∞
-
(CH3COO-
) is 349.8 and 40.1 ohm-1
cm-1
,
respectively.
Dissociation constant (Kd) for acetic acid could be given by the following formula
Kd=Cα2
/(1-α) --- eq6
where, C is the concentration. Graphically, dissociation constant can be determined by plotting
the graph of (α2
/1-α) vs. 1/Concentration.
Procedure i) Preparation of ~0.1N acetic acid (100 mL).
(Calculate, normality of acetic acid available in laboratory is 17.3N)
iii) Determination of exact normality of acetic acid.
Take 10 mL of acetic acid solution in a conical flask and titrate against standard solution of
sodium carbonate (0.1N) using methyl orange indicator.

2. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
Dr. Mithil Fal Desai
iv) Determination of equivalence conductance, degree of dissociation and dissociation
constant of acetic acid.
a) Measure the conductance of distilled water.
b) Pipette out 40 mL of 0.1 N acetic acid solution in a clean beaker. Dip the conductivity cell in
40 mL acetic acid solution whose cell constant is known and note down corresponding
conductance value. Subsequently, for other concentrations, pipette out the 20 ml of solution
from the beaker and add 20 mL of distilled water by a pipette. Repeat the procedure for five
dilutions to get five corresponding conductance readings. Calculate and tabulate the readings in
Table 1. Plot a graph of (α2
/1-α) vs. 1/Concentration to determine dissociation constant.
Observation.
Room temperature is =____ ◦C.
Conductance of distilled water ‘A’ =______ ohm-1
.
Cell constant =_____ cm-1
.
Table 1
Concentratio
n
of
CH3COOH
(N)
Measured
conductanc
e
(ohm-1
)
‘B’
Observed
Conductanc
e
(ohm-1
)
= B - A
Specific
conductanc
e (K)
(ohm-1
cm-1
)
Equivalent
conductanc
e (ᴧeq)
(ohm-1
cm2
g-equiv-1)
Degree
of
dissocia
tion
(α)
Dissociati
on
constant
(Kd)
α2
/(1-α) 1/C
0.1
0.05
0.025
0.0125
0.00625
Mean (Kd)
Results: i) Equivalent conductance of 0.1N acetic acid =_____ ohm-1
cm2
g-equiv-1
.
ii) Degree of dissociation for 0.1 N acetic acid =______.
ii) Dissociation constant graphically =______.
Further reading.
1) Essentials of Physical Chemistry, A. Bahl, B. S. Bahl, G. D. Tuli, S. Chand and Company Pvt.
Ltd.
2) Atkin’s Physical Chemistry, P. Atkins, J. de Paula, J. Keeler, Oxford, International.
3) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
4) Vogel’s textbook of quantitative chemical analysis, G.H. Jeffery, J. Bassett, J.
Mendham, R. C. Denney, Addison Wesley Longman Inc.
Questions
1) Explain Kohlrausch’s law.
2) What is the difference in measured conductance of strong electrolytes and weak electrolytes
at infinite dilution?
3) Compare the degree of dissociation of a weak electrolyte and a strong electrolyte.
4) Plot a graph of (α2
/1-α) vs. 1/Concentration to determine dissociation constant should pass
through the origin (0, 0). Why?