Solubility product:

Same consideration can also be applied to the dissociation of a weak base, BOH; in this case Kb will be the base dissociation constant.

BOH   ⇔      B⁺   + OH ^-

Initial concentration                         c          0          0

Equilibrium concentration             c (1-α) cα        cα

While c is the concentration of the base and α is the degree is dissociation.

K_b    = a_B+   + a_OH-/ a_BOH     

≈ [B ⁺] [OH ^- ]/ [BOH]

or K_b α ² c /   (1 - α)

Now consider a partial dissolution of a salt, MA, of low solubility, their equilibrium state could be written as

MA   ⇔      M⁺   + A^-

Equilibrium constant, K_s, can be written as

K_s   =   a_M⁺a_A^- /a_MA

Since, MA is in the form of a pure solid, therefore, a_MA = 1,

K_s    = a_M+a_A- ≈ [H ⁺] [A ^-]

where K_s is the solubility product.