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Concept of Solubility Product

There are many salts like NaCI, KNO3 and KI which are quite soluble in water. On the other hand, some salts like BaSO4, PbSO4, AND AgCI are very slightly soluble in water and are called sparingly soluble salts. Thus, if a sparingly soluble salt like AgCI is put in water, a saturated solution results and an equilibrium gets established between the ions of the dissolved salt and undissolved salt as :

                                                              AgCI(s)  <==>  Ag+ + CI -

Applying, the law of chemical equilibrium, we have

                                                           K  =  [ Ag+] [CI ]
                                                                    [ AgCI(s) ]
or                                   [ Ag+] [CI - ]  = K  X  [AgCI(s) ]

Since the concentration of a solid like AgCI(s) is always constant, no matter how much solid is present in contact with its solution, therefore, we have

                                        [ Ag+] [CI- ]  =  K  X  constant  = Ksp

where Ksp is known as solubility product and the expression [ Ag+] [CI - ] is known is ionic product.

In general, for any sparingly soluble salt  AxBy which dissociates to set up the equilibrium

                                      AxBy   <==>    X Ay+ + yBx-

the solubility product may be expressed as :

                                        Ksp  =  [ A y+ ]x [ Bx- ]y

where Ay+ and Bx- denote the positive and negative ions respectively while x and y represent the number of these ions in the formula of the electrolyte.

Hence, the solubility product a salt at a given temperature may be defined as the product of the concentration raised to a power equal to the number of times each ion occurs in the equation for the dissociation of the salt.

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