Common Ion Effect continued:

1

• Common Ion Effect (continued)

eg
Name two compounds that when added to a solution
will decrease the solubility of CaCl2.
NaCl or Ca(NO3)2 or any other soluble salt
containing Cl- or Ca2.
What about CaCO3? (hint check your
solubility table)
CaCO3 has low solubility, therefore, it does not
produce enough Ca2 to have any significant
effect.
2

• Qualitative Analysis

eg
A solution is known to contain Pb2, Cu2, and
Ca2 ions. Devise a method for separating these
ions. (assume concn gt 0.1 M)

• add KI and remove ppt. (I- is soluble with Cu2
  Ca2, but not with Pb2)

Pb2(aq) 2 I-(aq) ? PbI2(s)

• add K2S and remove ppt. (S2- is soluble with
  Ca2, but not with Cu2)

Cu2(aq) S2-(aq) ? CuS(s)

• Ca2 is left in the solution.

3
eg
Determine the identity of the unknown positive
ion in the following
Solution X

• Adding SO42- produces a ppt.

• Adding S2- does not produce a ppt.

• Adding OH- does not produce a ppt.

Answer Sr2
4
eg
If a 0.10 M solution of AgNO3 is used to detect
the presence of chloride ion in water samples,
what is the minimum concentration of Cl- that can
be detected?
(Ksp 1.8 x 10-10 for AgCl)
Ksp AgCl-
1.8 x 10-10 (0.10)Cl-
Cl- 1.8 x 10-9 M

• Thus in this example, if a precipitate forms, the
  Cl- in the water sample is greater than 1.8 x
  10-9