Chemistry
posted by Howard .
Solutions of sodium thiosulfate are used to dissolve unexposed AgBr (Ksp = 5.0 1013) in the developing process for blackandwhite film. What mass of AgBr can dissolve in 1.40 L of 0.425 M Na2S2O3? Ag+ reacts with S2O32 to form a complex ion.
Ag+(aq) + 2 S2O32(aq)> Ag(S2O3)23(aq) K = 2.9 10^13

AgBr(s) ==> Ag^+ + Br^
Ag^+ + 2S2O3^2 ==> Ag(S2O3)2
Add the two equations together for their sum and multiply the two k values (Ksp x Kformation). From the total equation you get Keq for the two. It should look like this.
..AgBr(s)+2S2O3^2 =>Ag(S2O3^2)2 + Br^
I.........0.425.........0...........0
C...........2x.........x...........x
E.........0.4252x......x...........x
Substitute the E line into the Keq expression (Keq = 14.5 if I punched my calculator right.
Solve x (a quadratic is not necessary) which gives M of the S2O3 solution. That x 1.40L gives mols and that x molar mass gives grams. Post your worki if you get stuck.