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March 27, 2017

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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 - ,

    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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