Calculation of Solubility Product from Solubility Data

Calculation of Solubility Product from Solubility Data

ChemiCalculation of solubility product from solubility data

Solubility is defined as the maximum amount of solute that can be dissolved in a solvent at equilibrium. The solubility product constant (K) describes the equilibrium between a solid and its constituent ions in a solution. The solubility product of a compound can be calculated from solubility data. This will be explained with the help of few worked out examples.

Example- 1: The solubility of silver chloride, AgCl, is 1.92 x 10-3 g L-1 at 25°C. Calculate the solubility product of AgCl at this temperature.

Solution:

In solution AgCl (s) ↔ Ag+ (aq) + Cl (aq)

Solubility in mol L-1 = [1.92 x 10-3 / 143.5] = 1.34 x 10-5

(Relative formula mass of AgCl = 143.51)

The concentration of each of the ions is equal to the solubility of the salt.

[Ag+] = [Cl] = 1.34 x 10-5

Ksp = [Ag+] = [Cl] = (1.34 x 10-5)2

= 1.8 x 10-10

Example- 2: At 250C the solubility of lead iodide, PbI2, is found to be 1.2 x 10-3 mol L-1. Calculate the solubility product, Ksp of lead iodide.

Solution

In solution, PbI2 (s) ↔ Pb2+ (aq) + 2I (aq).

The concentration, i.e. solubility, of the salt is equal to the concentration of Pb2+ ions. It can be seen that the concentration of iodide ion, I, is double that of lead ion, Pb2+.

[Pb2+] = 1.2 x 10-3 mol L-1.

[I] = 2 [Pb2+] = 2 x 1.2 x 10-3

= 2.4 x 10-3 mol L-1

Ksp = [Pb2+] [I]2 = (1.2 x 10-3) x (2.4 x 10-3)2

= 6.9 x 10-9.

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