Chemical equilibrium
LEC 03
03.08 Solubility product
What you can learn about
Solubility
Dissociation
Ionic conductivity
Ion mobility
Principle and tasks
The solubility of poorly soluble salts
is expressed as the solubility product,
i.e. the product of the concentrations
of cations and anions in the solution,
which are in equilibrium with the
solid salt. These concentrations can
be determined via conductivity
measurements.
What you need:
Digital conductivity meter, demo
13701.93
1
Conductivity / temperature probe
13701.01
1
Immersion thermostat, 100°C
46994.93
1
Accessory set for immersion thermostat
46994.02
1
Bath for thermostat, 6 l, Makrolon
08487.02
1
Rubber tubing, d i = 6 mm
39282.00
3
Hose clip, d = 8…12 mm
40996.01
4
Laboratory balance, with data output, 620 g
45023.93
1
Weighing dishes, 80 50 14 mm
45019.05
1
Magnetic stirrer, mini
47334.93
1
Magnetic stirrer bar, l = 30 mm
46299.02
2
Mortar with pestle
32603.00
2
Spoon
33398.00
1
Retort stand, h = 750 mm
37694.00
2
Right angle clamp
37697.00
3
Universal clamp
37715.00
3
Erlenmeyer flask, 100 ml, narrow neck
36118.00
4
Powder funnel, do = 65 mm
34472.00
1
Wash bottle, 500 ml
33931.00
1
Calcium carbonate, powder, 500 g
30052.50
1
Calcium fluoride, 250 g
31175.25
1
Conductivity standard solution
47070.02
1
Water, distilled, 5 l
31246.81
1
Solubility product
P3030801
46 Laboratory Experiments Chemistry
PHYWE Systeme GmbH & Co. KG · D - 37070 Göttingen
LEC
03.08
Solubility product
Related concepts
Solubility, dissociation, electrolytic conductance, activity.
Principle
The solubility of poorly soluble salts is expressed as the solubility product, i.e. the product of the concentration of cations and
anions in the solution which are in equilibrium with the solid salt.
These concentrations can be determined via conductivity measurements.
Tasks
Measure the conductivities of saturated aqueous solutions of
the salts calcium fluoride and calcium carbonate at 25 °C. With
the aid of tabulated ionic conductivities, calculate the solubility
products of the salts from their conductivities.
Equipment
Digital conductivity meter, demo
Conductivity / temperature probe
Immersion thermostat, 100°C
Accessory set for immersion thermostat
Bath for thermostat, 6 l, Makrolon
Rubber tubing, di = 6 mm
Hose clip, d = 8…12 mm
Precision balance, 620 g
Weighing dishes, 80 x 50 x 14 mm
Magnetic stirrer, mini
Magnetic stirrer bar, l = 30 mm
Mortar with pestle
Spoon, stainless steel
13701.93
13701.01
08492.93
08492.01
08487.02
39282.00
40996.01
48852.93
45019.05
47334.93
46299.02
32603.00
33398.00
1
1
1
1
1
3
4
1
1
1
2
2
1
Retort stand, h = 750 mm
Right angle clamp
Universal clamp
Erlenmeyer flask, 100 ml, narrow neck
Powder funnel, do = 65 mm
Wash bottle, 500 ml
Calcium carbonate, powder, 500 g
Calcium fluoride, 250 g
Conductivity standard solution
Water, distilled, 5 l
37694.00
37697.00
37715.00
36118.00
34472.00
33931.00
30052.50
31175.25
47070.02
31246.81
2
3
3
4
1
1
1
1
1
1
Set-up and procedure
Set up the experiment as shown in Fig. 1.
Dissolve 2 g of CaF2 and 2 g of CaCO3 each in 50 ml of distilled
water in separate Erlenmeyer flasks (pulverise the salts with the
mortar and pestle before weighing). In addition, prepare a third
Erlenmeyer flask with 50 ml of distilled water and a further one
with 50 ml of calibration standard solution. Put magnetic stirrer
bars in the two flasks with the salts, heat them to approximately 60 °C in the water bath, and subsequently stir them for 30 minutes at room temperature on a magnetic stirrer. To perform the
measurements, set the temperature-controlled bath to exactly
25 °C and temperature equilibrate the four Erlenmeyer flasks.
Use the calibration solution to calibrate the conductivity probe.
Measure the conductivities of the distilled water and the salt
solutions, whereby the measuring probe should only be
immersed in the clear solutions without stirring up the solid
phase. Thoroughly rinse the conductivity probe before each new
measurement. Substract the conductance value of the distilled
water from that of the salt solutions.
Fig. 1. Experimental set-up.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3030801
1
LEC
03.08
Solubility product
Theory and evaluation
A poorly soluble salt with the general form Mz+n+ Xz-n- (z+ and zare the charge numbers of the ions) forms anions and cations in
aqueous solution according to:
Mz+n+ Xz-n-
n+Mz+ + n-Xz-
(1)
Due to the extreme dilution, the equilibrium constant Ks can be
replaced by the solubility product L:
Ks ≈ (cMz+)n+ · (cXz-)n- = L
(2)
In contrast to the equilibrium constant, the solubility product is a
function of the concentration (salting-out effect).
The saturation concentration cs of a dissolved salt is as follows:
cS cz
cz
M
M
n
n
(4)
The saturation concentration can be determined conductiometrically. To do so, the ionic conductivities for infinite dilution M,
X are used (Table 1):
x = cs (n+M + n-X)
x
2
Specific conductance of the electrolyte solution
P3030801
cS x
n ¶M n ¶X
(6)
Substituting in equation (4), the solubility product can now be
calculated.
Table 1: Ionic conductivities at infinite dilution
Ion
Ca2+
Ionic conductivity in S · cm2 · mol-1
119.0
F-
55.4
CO32-
138.6
(3)
Substitution in equation (2) results in :
L = n+n+n-n-cs(n+ + n-)
Transposing according to cs, the following is obtained:
(5)
Data and results
Solubility products:
4.43 · 10-11 (lit.: 3.4 · 10-11) mol3 · l-3
CaF2:
CaCO3: 6.82 · 10-8 (lit.: 4.96 · 10-9) mol3 · l-3
The conductivity of solutions is strongly affected by even minute
traces of contaminants. Thus, for the measurement of the solubility product of calcium carbonate, the measured value is falsified by dissolved carbon dioxide from the air. As a result of the
formation of hydrogen carbonate, the solubility increases and
consequently the conductivity also.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen