For sparingly soluble salt AxBy: Ksp = $x^x$ * $y^y$ * s^(x+y), where s is the molar solubility. For AB type (AgCl): Ksp = $s^2$. For AB2 type (CaF2): Ksp = 4$s^3$. For A2B3 type (Bi2S3): Ksp = 108$s^5$. The common ion effect reduces solubility dramatically. AgCl in 0.1 M NaCl: s = Ksp/[Cl-] = 1.8 x 10^-10/0.1 = 1.8 x 10^-9 M vs 1.34 x 10^-5 M in pure water. When comparing solubility of different salts, you CANNOT compare Ksp values directly unless the salts have the same stoichiometric type (both AB or both AB2). For different types, calculate s explicitly and compare.