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>> Solubility Product-Part1
If to a given amount of solvent at a particular temperature, a solute is added gradually in increasing amounts, a stage is reached when some of the solute remains undissolved, no matter how long we wait or how vigorously we stir. The solution is then said to be saturated. A solution which remains in contact with undissolved solute is said to be saturated. At saturated stage, the quantity of the solute dissolved is always constant for the given amount of a particular solvent at a definite temperature.
In case the solute is an electrolyte, its ionisation occurs in solution and degree of dissociation depends on the concentration of dissolved electrolyte at a particular temperature. Thus, in a saturated solution of an electrolyte two equilibria exist and can be represented as:
AB ↔ AB ↔
Solid unionized ions
Applying the law of action to the ionic equilibrium,
Since the solution is saturated, the concentration of unionised molecules of the electrolyte is constant at a particular temperature, i.e., [AB] = K'= constant.
Hence, [A+] [B-] = K[AB] = KK = Ks (constant)
Ks is termed as the solubility product. It is defined as the product of the concentration of ions in a saturated solution of an electrolyte at a given temperature.
Consider, in general, the electrolyte of the type AxBy which is dissociated as:
AxBy ↔ xAy+ + yBx-
Applying law of mass action,
[Ay+]x[Bx-]y/[AxBy] = K
When the solution is saturated,
[AxBy] = K' (constant)
or [Ay+]x[Bx-]y = K [AxBy] = KK' = Ks (constant)
Thus, solubility product is defined as the product of concentrations of the ions raised to a power equal to the number of times the ions occur in the equation representing the dissociation of the electrolyte at a given temperature when the solution is saturated.
Note: Solubility product is not the ionic product under all conditions but only when the solution is saturated.