For a salt (M_mA_n): [ Q = [M^n+]^m [A^m-]^n ] Precipitation begins when (Q > K_sp). The threshold concentration of (A^m-) needed is: [ [A^m-] > \left( \fracK_sp[M^n+]^m \right)^1/n ]
As the precipitating agent is added drop-wise, you calculate the reaction quotient ( Qspcap Q sub s p end-sub ) for both potential solids to see which reaches its Kspcap K sub s p end-sub first. The substance with the lower Kspcap K sub s p end-sub fractional precipitation pogil answer key