Silver bromide, AgBr (s), is an essential reagent inblack and white film developing. It is,

however, only sparingly soluble in water. AgBr (s) hasK = 5.0 × 10-13, making it difficult to

rinse AgBr from the film negative with water.

Instead, excess AgBr is removed by an aqueous solution of sodiumthiosulfate (Na2S2O3), which forms the complex ion Ag(S2O3)23-:

Ag+ (aq) + 2 S2O32- (aq) Ag(S2O3)23-(aq) Kf = 4.7 × 10+13 a) To see how this helps,first determine the molar solubility of AgBr (s) inwater.

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b) The large formation constant of Ag(S2O3)23- (aq)means that almost all of the silver is complexed with thethiosulfate. To determine how much, calculate the [Ag+] atequilibrium in a 1.0 L solution that initially contains 0.001 Msilver ions and 0.200 M sodium thiosulfate. Do so by setting up anICE table and assuming that x is small compared to 0.200(but not 0.001). Show that this results in a value ofx = [Ag(S2O3)23-] = 0.001. In other words, all ofthe Ag+ complexes.

c) Your result from part b suggests that no Ag+ remains insolution. This obviously can’t be correct, since it would result inan infinitely large reaction quotient that wouldn’t be equal to anadmittedly large equilibrium constant. To determine the correct[Ag+], use the equilibrium concentrations determined in part b asyour initial concentrations in a new ICE table. Then useKf to determine [Ag+]. [This approach is called astoichiometric shift and is useful when a reaction startswith only reactants but goes almost to completion. Is essence, weare approaching the equilibrium from the direction of allAg(S2O3)23- and no Ag+ as opposed to the direction of all Ag+ andno Ag(S2O3)23-.]

d) Before we determine the solubility of AgBr (s) in athiosulfate solution, we need to know the appropriate equilibriumconstant. To find it, determine the value of the equilibriumconstant for the reaction:

AgBr (s) + 2 S2O32- (aq) <-->Ag(S2O3)23-(aq) + Br - (aq) Kc = ??

e) Finally, calculate the molar solubility of AgBr (s)in 1.0 M sodium thiosulfate. In other words, what is [Ag(S2O3)23-]for the reaction given in part d when the initial concentration ofthiosulfate is 1.0 M?