The Ksp-Solubility Conundrum

The calculation of solubility from Ksp values, and its converse have long been a part of general college chemistry textbooks. The agreement between actual solubility and simple calculations of solubility from Ksp values has been questioned many times. Incomplete dissociation, hydrolysis reactions, complex ion formation, ion pair formation and activity coefficients conspire to make such calculations crude at best.The authors sought to find those cases of partly soluble substances for which simple calculations such as Ksp=S2, and Ksp=4S3 can realistically be used for computing solubility from Ksp and the reverse calculation. Empirical solubility data are compared with solubilities calculated from reliable Ksp values that have been adjusted for activity coefficients. A table of 18 substances is suggested for textbook use.