SOLUBILITY AND THE SOLUBILITY PRODUCT CONSTANT
Since the solubility product constant contains information about the maximum concentration of
ions in the solution of a particular salt, it should come as no surprise that Ksp is related to the salt’s
solubility, the maximum amount that can be dissolved in the solvent. For ionic compounds that
dissociate into constituent ions, the way these two quantities relate to each other depends on the
stoichiometry of the compound.
For the general dissolution and dissociation of the type
AmBn (s) → mAn+ (aq) + nBm− (aq),we have already determined the relationship Ksp = [An+]m[Bm−]n. Relating that to the solubility of
AmBn, we need to keep in mind that for every x moles of AmBn that dissolve, mx moles of An+ and nx
moles of Bm− will result. Thus if x is the molar solubility of the salt, then:
So for salts of the form MX, e.g., AgCl, the reaction is:
MX (s) → M+ (aq) + X− (aq),and Ksp = [M+][X−] = x^2 . Whereas for salts of the form MX 2 , e.g., CaCl 2 , the reaction is:
Every slightly soluble salt of the general formula MX will have a Ksp = x^2 , where x is the
molar solubility.
Every slightly soluble salt of the general formula MX 2 will have a Ksp = 4x^3 , where x is the
molar solubility.
Every slightly soluble salt of the general formula MX 3 will have a Ksp = 27x^4 , where x is
the molar solubility.