Ksp, Activities and Activity Coefficients

Definition of Ksp and K'sp For a solution containing solid AgCl precipitate, AgCl(s) --> Ag+(aq) + Cl-(aq) For a solution containing solid PbCl2 precipitate, PbCl2(s) --> Pb2+(aq) + 2Cl-(aq) Note how the moles in the chemical reactions become coefficients in the equilibrium expressions. Notes: The lower case Roman "a" is activity; the Greek gamma is activity coefficient. Activity coefficients are computed from the ionic strength and ionic size using the Debye-Huckel equation. The Debye-Huckel equation is only good for ionic strengths less than 0.1. Most examples of Ksp calculations use Ksp in place of Ksp'and are not "corrected" for ionic strength. To correct for ionic strength, simply compute Ksp' and use it in place of the regular Ksp.