Page 55 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

QUANTITATIVE EFFECTS OF A COMMON ION 2.7

ion. Thus the solubility of a Salt MA in the presence of a relatively large amount
of the common M' ions,* supplied by a second Salt MB, follows from the
definition of solubility products:

The solubility of the Salt is represented by the [A-] which it furnishes in
solution. It is clear that the addition of a common ion will decrease the solubility
of the salt.
Example 4. Calculate the solubility of silver chloride in (a) 0.001 M and (b)
0.01 M sodium chloride solutions respectively (K,(A,Cl) = 1.1 x 10-'O mol2 L- 2).

1.05
In a saturated solution of silver chloride [Cl-] = dm= x
10- mol L- '; this may be neglected in comparison with the excess of Cl- ions
added.
For (a) [Cl-] = 1 x [Ag'] = 1.1 x 10-'0/1 x

For (b) [Cl-] = 1 x [Ag'] = 1.1 x 10-'0/1 x

Thus the solubility is decreased 100 times in 0.001M sodium chloride and
1000 times in 0.01 M sodium chloride. Similar results are obtained for 0.001 M
and 0.01 M silver nitrate solutions.
Example 5. Calculate the solubilities of silver chromate in 0.001 M and
0.01 M silver nitrate solutions, and in 0.001 M and 0.01 M potassium chromate
solutions (Ag2Cr0,: K, = 1.7 x 10-l2 mol3 L-3, solubility in water = 7.5 x
10-~m01~-~).
[Ag+I2 x [CrO:-] = 1.7 x 10-l2

or [Cr0:-] = 1.7 x 10-'~/[Ag']~
For 0.001 M silver nitrate solution: [Ag'] = 1 x

For 0.01 M silver nitrate solution: [Ag'] = 1 x
[CrOz-] = 1.7 x 10-12/1 x = 1.7 x mol L-'
The solubility product equation gives:
[~g+] = J1.7 x 10-'2/[CrO:-]
For [CrO:-] = 0.001, [Ag'] = J1.7 x 10-12/1 x

*This enables us to neglect the concentration of M+ ions supplied by the sparingly soluble salt
itself, and thus to simplify the calculation.