Ag++Br−⇌AgBr Precipitation starts when ionic product just exceeds solubility product Ksp =[Ag+][Br−][Br−]=[Ag+]Ksp=0.055×10−13=10−11 i.e., precipitation just starts when 10−11 moles of KBr is added to 1 L of AgNO3 solution. No. of moles of KBr to be added ∴ weight of KBr to be added =10−11×120=1.2×10−9 g