LAB

Theory
If Br- ions are added to any solution containing Ag+ ions, AgBr cannot precipitate until the product of the ion concentrations, (Ag+) (Br-), exceeds the solubility product constant, Ksp, for AgBr. A solution that contains NH3 molecules in high concentration can only contain Ag+ ions in very low concentration because NH3 molecules and Ag+ ions unite to form a complex with a large formation constant. If we represent the formation of the complex by the equation,
Ag+ + nNH3 = Ag(NH3)n+ (1)
Then the expression for the formation constant (equilibrium constant with the concentration of the complex in the numerator) is
Kf = [Ag(NH3)n+] (2)
[Ag+] [NH3]n

Since the formation constant of the aminesilver complex has a large numerical value, a high concentration of Br- ions must be established in order to produce a AgBr precipitate from a solution of “ammoniacal silver nitrate...