AgBr⇌Ag++Br− Ksp=[Ag+][Br−] For precipitation to occur Ionic product > Solubility product [Br−]=
Ksp
[Ag+]
=
5×10−13
0.05
=10−11 i.e., precipitation just starts when 10−11 moles of KBr is added to ILAgNO3 solution ∴ Number of moles of Br− needed from KBr=10−11 ∴ Mass of KBr=10−11×120=1.2×10−9g