Solubility product of silver bromide is 5.0x10-13. The quantity of potassium bromide (molar mass taken as 120 g mol–1) to be added to 1 litre of 0.05 M solution of silver nitrate to start the precipitation of AgBr is
[Ag+][Br-] > Ksp[AgBr]
[Br-]min = (5 x 10-13)/0.05 = 10-11 M
Mass of potassium bromide needed = 10-11 x 120
= 1.2 x 10-9 g
The correct option is C.