Question

Calculate how many moles of Na2S2O3 must be added to dissolve 0.020 mol AgBr in 1.0 L of water at 25°C (assume constant volume).

AgBr(s) ↔ Ag+(aq) + Br−(aq)
Ksp = 3.3 x 10^−13

Ag+(aq) + 2S2O32−(aq) ↔ [Ag(S2O3)2]3−(aq)
Kf = 2.0 x 10^13

(1) 0.0078 mol (2) 0.048 mol (3) 0.040 mol (4) 0.024 mol (5) 0.016 mol

Answer 1

Your Ksp for AgBr is eqn 1.
Your Kf for Ag(S2O3^-2)^3- is eqn 2.

Add equn 1 to eqn 2 to get
AgBr(s) + 2(S2O3)^3- ==> Br^- + [Ag(S2O3)2]^3-
Write the equilibrium expressiion for that which equals Ksp*Kf.
Then substitute 0.02 for Br^- and 0.02 for [Ag(S2O3)]^3- and solve for (S2O3)^2-. Remember AgBr(s) does not appear in the equilibrium expression. Looks like (1) is the answer but you should confirm that yourself.