Solubility product of silver bromide is 5.0 × 10⁻¹³ . 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⁻ → AgBr
Precipitation starts when ionic product just exceeds solubility product
Ksp = [Ag⁺][Br⁻]
[Br⁻] = Ksp/[Ag⁺] = 5 × 10⁻¹³/0.05 = 10⁻¹¹
i.e., precipitation just starts when 10⁻¹¹ moles of KBr is added to 1L of AgNO₃ solution.
No. of moles of KBr to be added = 10⁻¹¹
Weight of KBr to be added = 10⁻¹¹ × 120 = 1.2 × 10⁻⁹g