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(i) Common ion effect
(ii) The relation between `K_(sp)` and solubility (S) of a sparingly soluble salt `BaSO_(4)`.

Explanation

(i) Common ion effect: The solubility of an electrolyte in water decreases by the addition of another electrolyte which has one ion common with the electrolyte.
For example the solubility of NaCl decreases on addition of HCl of solution of Nacl in water which is known as salting out of water.
Acetice acid as weak acid and it undergoes partial ionization as follows:
`CH_(3)COOHhArrCH_(3)COO^(-)+H^(+)`
In case of such electrolytes there exists an equilibrium in between ions and molecular form. In such a condition the addition of electrolyte with common ions shift the equilibrium in backward direction with causes the decreases in solubility.
Significance:
1. Common ion effect is an important pohenomenon in qualitative analysiis.
2. The common ion effect principle is also used in controlling the `H^(+)` ion concentration in buffer solution.
(ii) the relation between `K_(sp)` and solubility (S) of a sparingly soluble salt `BaSO_(4)`: The sparling soluble salt like `BaSO_(4)`, when added to water form saturated solution. There exists an equilibrium between undissolved solid and the ions in saturated solution.
`BaSO_(4)hArrBa^(2+)+SO_(4)^(2-)`
The equilibrium constant
`K=([Ba^(2+)][SO_(4)^(2-)])/([BaSO_(4)])`
`K=([Ba^(2+)][SO_(4)^(2-)])/(K_(1))`
(`:.` Activity of solid `BaSO_(4)` is constant)
`KxxK_(1)=[Ba^(2+][SO_(4)^(2-)]` or
`K_(sp)=[Ba^(2+)][SO_(4)^(2-)]`
The new constant `K_(sp)` is called solubility product. If S `"lit"^(-1)` is the solubility of `BaSO_(4)`
`underset(s)(BaSO_(4)(s))hArr underset(s)(Ba^(2+)+underset(s)(SO_(4)^(2-))`
`K_(sp)=sxxs=s^(2)`
or solubility of `BaSO_(4)s=sqrt(K_(sp))`