Egyptian academy of engineering and advanced technology
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Ministry of higher education.
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Experiment 5
determination of dissociation constant of a weak electrolyte (Ka)
Concepts: will be discussed in details at the laboratory.
Theory
For weak electrolytes, the variation of conductance with solute concentration is essentially due to
change in the number of ions as determined by the extent of dissociation. The degree of
dissociation α at a definite concentration C, may be obtained from the relation:
α = Ʌequiv. /Ʌo
eqn. (1)
For weak electrolytes, Ʌo can be calculated from Kohlrausch’s law:
Ʌo = λ+ + λ-
eqn. (2)
The relation between α & C & Ka of a weak acid can be obtained using Ostwald law of dilution:
Ka = α2 C / 1-α
eqn. (3)
This law can be written in the form
C Ʌequiv/ Ka Ʌo = (1/Ʌo) - (1/Ʌequiv.)
eqn. (4)
Procedure:
1- Prepare 500 ml of exactly 0.4 N acetic acid solution. From this stock solution prepare 100
ml of each 0.1 N, 0.2 N, 0. 05 N, 0. 025 N and 0. 01 N solutions.
2- Determine the specific conductivity of each solution using conductivity meter.
3- Calculate the equivalent conductivity of each solution.
Ʌequiv. = Κ/C(N) = Κ *1000/C =
4- tabulate your results in the following table
S.cm2. equiv-1 eqn. (5)
Egyptian academy of engineering and advanced technology
Affiliated to Ministry of military production.
Ministry of higher education.
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Κ (S.cm-1)
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Ʌequiv S.cm2.equiv-1
1/Ʌ equiv C*Ʌequiv.
α
0.02
0.1
0.05
0.01
0.005
0.001
5- Plot 1/Ʌequiv vs. C *Ʌequiv., draw the fittest straight line. from the intercept estimate Ʌo
and the slope equals 1/Ʌo2 * Ka. calculate Ka
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