Theoritical principles in qualitative analysis
(Hydrogen Sulphide scheme)
Solubility and Solubility Product :
The
maximum amount of solute that can be dissolved in a definite amount of solvent
at a particular temperature is called the solubility.
For
example the solubility of NaCl in water is 0.359 g/ml or simply 35.9 g/100ml at
25 degree centigrade and that of AgCl in water is only 520 micro gram/100 g at
50 degree centigrade. Thus we say AgCl is a sparingly soluble salt as it is
soluble to a very less extent.
AgCl (s) ⇌ Ag + + Cl -
AgCl (s) stands here for solid AgCl
Applying the law of chemical equilibium,
K = [Ag+ ] . [Cl -] / [AgCl (s)] ---------- equation 1
Since AgCl is sparingly soluble (dissolves in water to a very small extent ), its saturated solution is prepared easily. The point here to note is that, what ever amount of a spairingly soluble salt dissolves, it is completely ionised . In other words, a saturated solution has its maximum ion conc. in the solution. Also the conc. of solid AgCl (left undissolved and settled down) becomes fixed or simply a constant. Thus from equation.1
=> K [AgCl] = const. = Ksp = [Ag+ ] [Cl -]
The constant Ksp is called the solubility product.
All we need to create in our mind is that, the solubility product of a salt is obtained when the solution of that salt is saturated and has the maximum ion concentration. This can be applied to any type of salt.
In general for a salt of the type , AxBy ,
AxBy → XAy+ + YB x-
Ksp = [Ay+ ] x . [B x-] y
Thus the solubility product of a salt at a given temperature may be difined as :
" The product of molar conc. of ions (formed in the saturated solution at a given temp.) raised to the power equal to the number of times each ions occurs in the equation of solubility equilibrium .
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