Question

Write the dissociation reaction and the K_sp expression for Aluminum Phosphate (AlPO₄) dissolving in water.

Answer 1

The dissociation of the sparingly soluble salt, aluminium phosphate, in water can be shown as below.

AlPO₄(s) Al³⁺(aq) + PO₄^3-(aq)

Initial concentration (M): s 0 0

Equilibrium concentration (M): s-sx sx sx; where 'x' is the degree of dissociaion of AlPO₄.

The equilibrium constant for the dissociation reaction above above is

K = [Al³⁺]_e[PO₄^3-]_e / [AlPO₄], where the suffix 'e' denotes equilibrium.

K [AlPO₄] = [Al³⁺]_e[PO₄^3-]_e

Here, only a small part of AlPO₄ is soluble in water, hence its concentration is constant.

i.e. K [AlPO₄] = (K_sp)_AlPO4.

Therefore, (K_sp)_AlPO4 = [Al³⁺]_e[PO₄^3-]_e

i.e. (K_sp)_AlPO4 = (sx) * (sx)

Whatever part of salt is soluble in water, that part completely (100%) dissociates into corresponding ions, i.e. x = 1.

Therefore, (K_sp)_AlPO4 = s * s, i.e. s².