Equilibrium

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

  1. 6.2 x 10-5 g
  2. 5.0 x 10-8 g
  3. 1.2 x 10-9 g
  4. 1.2 x 10-10 g

Answer

For precipitation

[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.