Page 111 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

2-
+
Q2: What are the molar concentrations of Ag and CrO4 in a saturated
-12
solution of Ag2CrO4 at 25 °C ? Ksp (Ag2CrO4)= 1.9 x 10
Solution:

+ 2-
Ag CrO ↔ 2 Ag + CrO
2 4 4
X 2X X
+ 2 2-
K = [Ag ] [CrO ]
sp 4
2 3 -12
K = (2X) (X) = 4 X = 1.9 x 10
sp
-5
X = 7.8 x 10 M
+ -5 -4
[Ag ] = 2 [7.8 x 10 ] = 1.6 x 10 M
2- -5
[CrO ]= 7.8 x 10 M
4
Significance of Ksp

(1) For a ppt. of a sparingly soluble salt to be formed in a solution

+ -
(for example AgCl) : the ionic product (Q) of its respective ions, [Ag ][Cl ], must
exceed the K of the salt (AgCl).
sp
+ -
Q (Product of molar concentration of ions) = [Ag ][Cl ]
➢ When the ionic product (Q) is less than the solubility product (K ),the solution
sp
is unsaturated.
+ -
➢ If Q just equals K , all Ag and Cl will remain in solution (saturated solution)
sp
.

➢ When (Q) exceeds K , the solution is supersaturated and precipitation will
sp
occur.

So, Q must be  K of AgCl for a ppt. of AgCl to be formed
sp
The ppt. is formed when the product of ions concentrations exceed the solubility
product at given temperature.

In quantitative analysis, excess precipitating agent is always employed to ensure
complete precipitation.

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