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8.CHEMICAL EQUILIBRIUM eLearn.Punjab

Law of mass action applied to the dissociation of PbSO4 gives equilibrium constant Kc

K = [Pb2+ ][SO42− ]
[PbSO4 ]
c

Being a sparingly soluble salt the concentration of lead sulphate (PbSO4) almost remains
constant. Bring [PbSO4] on L.H.S. with Kc

Kc[PbSO4 ] = [Pb2+ ][SO42- ]

if Kc[PbSO4 ] = Ksp

then Ksp =[Pb2+ (aq)][SO42-(aq)]= 1.6x10-8 at 25oC

Ksp is called the solubility product of PbSO4. It is the product of molar solubilities of two ions
at equilibrium stage.

Similarly,for PbCl2 Ksp =[Pb2+ (aq)][Cl- (aq)]2

Ksp is usually a very small quantity at room temperature. The value of Ksp is temperature
dependent. For a general, sparingly soluble substance, AxBy.

AxBy  xA+y +yB-x

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

So, the solubility product is the product of the concentrations of ions raised to an exponent
equal to the co-efficient of the balanced equation. The value of Ksp is a measure of how far to the
right dissolution proceeds at equilibrium i.e. saturation. The following Table (8.10) shows us the Ksp
values of slightly soluble ionic compounds.
Smaller the value of Ksp, lesser the capability to be dissociated.

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