Solubility product constant and its applications

Solubility Product Constant

 ---> Solubility Product is the product of the concentration of ions of the electrolyte each raised to the power of their coefficients in the balanced chemical equation in a saturated solution.

---> It has a constant value for an electrolyte at a constant temperature.

---> Solubility product constant is denoted by (Ksp)

---> To understand this concept, lets take one example:

• Consider the saturated solution of FeS in water. The equilibrium is represented by....

• The equilibrium constant for above reaction is.....

• FeS is in solid state and it will not affect the equilibrium constant as it is in its standard state. So, the equilibrium constant is expressed as Ksp (Solubility Product Constant)

• Where [Fe ๋²] and [S¯²] represent the concentration of ions in moles/liter. Here the unit of Ksp is (moles/liter)² 
• The general precipitation reaction is written as 

• The unit of Ksp is (moles/liter)^a+b
• ๋This is called mathematical statement of solubility product constant.

Application   

• The conecpt of solubility product is very useful in explaining many phenomena. It is commonly used in prediciting the solubility and precipitation of compounds in analytical chemistry that are given below

1. calculation of solubility of the slightly soluble ions

---> knowing the solubility product of a sparingly soluble salt like AgCl, PbI2, BaSO4, etc. the solubility of the salt can be calculated

2. Predicting thr precipitation of ions in the reaction:

---> knowing the solubility product of salt it is possible to predict whether a precipitate will be formed or not.

---> MCI - Molar Concentration of Ions

--->Ksp - Solubility Product Constant

---> Product of MCI < Ksp ---> unsaturated solution (No precipitation)

---> Product of MCI < Ksp = saturated solution 

---> Product of MCI > Ksp ---> Super saturated solution which gives precipitates on standing

3. In Inorganic Qualitative Analysis:

----> For a mixture to be separated by fractional precipitation the Ksp of both substance should be different and the concentration of ions to be separated must not be too different.