Applications of conductivity measurements: Calculating the solubility
product of a partially soluble salt (Lab report)
Theory
The solubility of poorly soluble salts is expressed as the solubility
product. That is the product of the concentration of ions in the solution which
are in equilibrium with the solid ion. These concentrations can be determined
via conductivity measurements. In this practical PbSO4 is used.
PbSO4 ⇌ Pb2+ + SO42-
Assume concentration of solid is a constant.
Concentration of Pb2+= concentration of SO42- =
C
\ Ksp =
[Pb2+aq] [SO42-aq]
Ksp = C2
The measurement of the specific conductivity, K of the saturated
solution leads to a value of the concentration.
i.e.
C = K/ Λ0
As, Ksp = C2
Ksp
= (K/ Λ0)2
Λ0 for PbSO4 = 3.02 × 10-2 m2Smol-
Λ0 = Molar conductivity at infinite dilution
Procedure
About 2g of finely powdered PbSO4 was measured using the
electric balance into a clean, dry watch glass.
It was added to a clean beaker. The watch glass was washed with
distilled water and that washed water was also added to the beaker.
To remove impurities it was shaken by adding distilled water. It was
kept aside for a while, for the salt to be collected at the bottom of the
beaker. Then the upper layer was decanted and repeated this step for several
times.
An empty reagent bottle was kept in a water bath at 240C in order
to maintain the temperature of the reagent bottle at 250C.
Then the well- washed salt was added to the reagent bottle and added
100mL of distilled water.
After awhile without disturbing the solid, the clear upper solution was
decanted into a clean beaker and the conductivity was measured.
Results
K of distilled water at 250C = 4.2 µS/cm
K of PbSO4 at 250C = 39.2 µS/cm
Calculations
Corrected conductivity of PbSO4 = (39.2 – 4.2) µS/cm
= 35.0 µS/cm
= 35.0 × 10-2 Sm-
C = K/ Λ0
C = 35.0 × 10-2 Sm- / 3.02 × 10-2 m2Smol-
C = 0.11589 molm-3
C = 0.11589 × 10-3 moldm-3
PbSO4 ⇌ Pb2+ + SO42-
Ksp = [Pb2+aq] [SO42-aq]
Ksp = C2
Ksp = (0.11589 × 10-3 moldm-3)2
=
1.3430 × 10-8 mol2 dm-6
Conclusion
Exact given Ksp at 250C is 1.6 × 10-8 mol2 dm-6.
Practically obtained Ksp at 250C is 1.3430× 10-8 mol2 dm-6
As the both values are closer we can use conductivity measurements in
order to calculated KSP.
Discussion
The precipitate was washed in order to remove impurities that may affect
the results. It should be washed several times to obtain a well- washed precipitate.
During this process, precipitate can be also removed. Therefore all 2g of the
sample wouldn’t be left.
When maintaining the temperature of 250C, the water
bath was kept at a temperature about 240C. this is because we
measure the temperature of the water bath assuming the temperature inside the
reagent bottle is at 250C. But actually the temperature of the
solution inside would be slightly higher than the temperature of the water bath
as the glass of the reagent bottle separates both. Therefore the temperature of
the water bath was kept at lower temperature to obtain the required temperature
for the solution.
Reagent bottle should be shaken time to time when it was kept in the
water bath. This is to increase the dissociation and reach the equilibrium. If
not, we might have not measured the conductivity at the equilibrium and that
will alter the true Ksp value.
The calculated Ksp value is slightly smaller than the exact given Ksp
value due to errors occurred during the experiment. Mainly the personal errors.
Such as temperature maintaining errors, measuring errors and personal
carelessness.
When preparing the solutions, the distilled water was added. Distilled
water also contains ions it self. Therefore additional conductivity from the
distilled water would be raised in the solution. For this reason the
conductivity of the solution is corrected by deducting the conductivity of
water.
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