ELECTRICAL CONDUCTANCE
PURPOSE
The purpose is to determine the conductances
of solutions of a salt, strong acid, weak acid, and slightly soluble electrolyte.
From the data the ionization constant of the
weak acid and solubility product constant of the slightly soluble salt
may be calculated.
DISCUSSION
Conductance is defined as the reciprocal of resistance.

It is expressed as "reciprocal ohms"
or "mho" ("ohm" spelled backward). "Specific conductance"
is ohm-1 cm -1
Specific conductance decreases as the concentration
of ions decreases.
"Equivalent conductance", L,
is defined as

where C is the concentration.
The equivalent conductance in a solution in which
the ions are far enough apart not to interact (infinite dilution) is known
as L0, equivalent
conductance at infinite dilution. The ions act independently, and L0
is the sum of the limiting conductances of each ion.

(Values of single ion conductances may be found
in the CRC Handbook of Chemistry and Physics.
L0 may
be determined by plotting L vs
and
extrapolating to zero concentration. However, this is not successful for
a weak electrolyte because the degree of ionization increases with dilution
and the curve is not linear.
L0
for acetic acid may be determined from l0
of
the ions.
For a weak electrolyte the degree of dissociation
is

Consider a weak acid:

Concentrations:

For the slightly soluble salt

However, since the conductance, L, is
very small, the conductance of pure water should be subtracted. Thus

The solubility product constant can then be calculated
from the concentration of the salt.
EQUIPMENT AND CHEMICALS
A.C. conductance bridge (YSI or Beckman), conductivity
cells, 0.100 M NaCl, 0.100 M HCL, 1.0 M CH3COOH,
saturated solution of PbSO4. (Solution concentrations need not
be exactly 0.1000 M, but should be known to three significant figures.)
DIRECTIONS
- Read the instructions for using conductance
bridges supplied by the manufacturer. Both instruments are essentially
the same. You have a choice of two A.C. frequencies. Multipliers and scale
dials are adjusted to give a minimum on the null meter (Beckman) or wide
shadow (YSI). At that point the conductivity is equal to

- Extreme care must be made in making solutions
and successive dilutions. The strong electrolytes are diluted by one-half
so that you have these solutions: 0.025, 0.0125, 0.00625 M, 0.00313
M. Acetic acid is diluted similarly.
- Measure conductance of distilled water first.
Then measure solution conductances, starting with most dilute solution.
After each reading wash the cell with portions of the next solution.
- Wash some solid PbSO4 with successive
portions of distilled water to remove any soluble impurities. Then determine
the specific conductance of a saturated PbSO4 solution. For
PbSO4,
- Ordinary distilled water is not satisfactory
since it has too high a conductance, mostly due to dissolved CO2.
Much better water can be obtained by boiling distilled water to free CO2
and capping a full bottle while it is hot. Its specific conductance should
be
or less. (200,00 ohm resistance).
- Since conductivity is temperature dependent,
the experiment may be run in a constant temperature water bath.
UTILIZATION OF DATA
- For each series of solution graph L
vs .
If a straight line is obtained use a least squares program on the computer
to determine L0
(the intercept). Compare results for strong and weak electrolytes. If the
points are scattered make more measurements to define a smooth curve. Compare L0
for each strong
electrolyte with accepted values.
- For CH3COOH, calculate Ka.
- For PbSO4 calculate Ksp.
SAMPLE CALCULATIONS
Experiment: Exp. 9.4 Electrical
Conductance
Data:
conc
(mol / L)
|
L
( )
|
0.05
|
111.8
|
0.025
|
105.6
|
0.0125
|
104.0
|
0.00625
|
110.4
|
0.003125
|
121.4
|
Calculations:

|