Homework Help: Chemistry

Posted by Howard on Monday, March 18, 2013 at 12:32am.

Solutions of sodium thiosulfate are used to dissolve unexposed AgBr (Ksp = 5.0 10-13) in the developing process for black-and-white 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

Chemistry - DrBob222, Monday, March 18, 2013 at 1:12am
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.........0.425-2x......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.

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