Inorganic and Applied Chemistry

Download free books at BookBooN.com

Inorganic and Applied Chemistry

Example 4- A:

Calculation of solubility of PbSO 4

We wish to calculate the solubility of lead sulphate PbSO 4 at 25 ºC. At 25 ºC PbSO 4 has a solubility

constant of 1.3 · 10-8 M^2. When solid PbSO 4 dissolves initially the system only contains PbSO 4 (s) and H 2 O

but as time goes the solid specie will dissolve following the reaction scheme below:

PbSO 4 (s)  Pb2+(aq) + SO 4 2-(aq)

Due to the equilibrium reaction scheme the equilibrium expression may be written as:

Ksp = [Pb2+]·[SO 4 2-]

To determine the solubility of PbSO 4 it is necessary first to find the equilibrium concentrations of Pb2+ and

SO 4 2-. This may be done by specifying the initial concentrations (prior to any dissolution of lead sulphate)

followed by a definition of the changes that are needed in order to reach equilibrium. As we in this case do

not know the solubility vi assume that x moles/L of the solid specie is to be dissolved in order to reach

equilibrium. The scheme of reaction indicates 1:1 stoichiometry which is why we have:

x moles/L PbSO 4 (s)  x moles/L Pb2+(aq) + x moles/L SO 4 2-(aq)

By substitution of the equilibrium concentrations into the expression for Ksp we achieve:

1.3 · 10-8 M^2 = Ksp = [Pb2+]·[SO 4 2-] = x · x  x = Ksp

By which we get x = 1.1· 10-4 M. Thus, the solubility of lead sulphate PbSO 4 at 25 ºC is 1.1· 10-4 moles/L.

It is very important to distinguish between solubility of a solid specie and the product of solubility. Whereas

the product of solubility is an equilibrium constant and thereby only has one value at a certain temperature,

solubility is an equilibrium position which has an infinity of possible values at a given temperature

depending on other factors such as the presence on foreign ions and more. The solubility product and the

solubility of ions do also have different units which is why they cannot be readily compared.

4.1.1 Relative solubility

A solid species solubility constant Ksp gives important information about the species solubility. This is very

convenient in a situation where only the solid specie is to be dissolved. Ksp values are often easy to find in

literature. Are several solid species about to go into solution, the solubility product cannot be used in similar

manner as earlier described. In principle one may imagine two situations, both of which we are going to look

further into in the following examples. In one of the situations two or more solid species are about to go into

solution where the solid species produce the same amount of ions. In the other situation there is likewise two

or more different solid species but this time they produce a different amount of ions.

Equilibrium