[nernst]

Here is the question I'm working on:

How many moles of Na₂S₂O₃ must be added to dissolve .030 mol AgBr in 1.0 L of water?

Kf = 2E13
Ksp= 3.3E-13

balanced equation :

AgBr + 2S₂O₃^2- <-----> Ag(S₂O₃)^3- + Br-

I                                          0                     0
C                  ?                     +.030                +.030
E          .01168                .030                   .030

Now, I set up an I.C.E table for it. This is how far I get, and then I'm rather confused about how to proceed?

[plankk]

You are really close to correct answer. 

Let's think. As your ICE table "says" in equilibrium should be .01168 mol per litr of S₂O₃^2-. Also in that state should be .030 mol per litr of [Ag(S₂O₃)]^3-. How many moles of S₂O₃^2- is needed to create .030 mol of [Ag(S₂O₃)]^3- (remeber that in the task they ask for 1.0 L)? The sum of that value and that in equilibrium state is the whole demand for Na₂S₂O₃.

[nernst]

You are really close to correct answer. 

Let's think. As your ICE table "says" in equilibrium should be .01168 mol per litr of S₂O₃^2-. Also in that state should be .030 mol per litr of [Ag(S₂O₃)]^3-. How many moles of S₂O₃^2- is needed to create .030 mol of [Ag(S₂O₃)]^3- (remeber that in the task they ask for 1.0 L)? The sum of that value and that in equilibrium state is the whole demand for Na₂S₂O₃.

What I had trouble with was working backwards to calculate the moles.

[plankk]

So let's try step by step...

Answer for that question: how many moles of S₂O₃^2- is needed to create ,030 mol of [Ag(S₂O₃)]^3-?

[nernst]

So let's try step by step...

Answer for that question: how many moles of S₂O₃^2- is needed to create ,030 mol of [Ag(S₂O₃)]^3-?

2

[plankk]

2 yes, but to create 1 mol of complex. We want to have .030 mol of complex so we need .060 mol of S₂O₃^2-.

In equilibirum state the concentartion of thiosulfate (as you count) is .01168 mol/l, so in 1 litr of water should be .01168 mol of S₂O₃^2-.

As you see the thiosulfate occures as a free anion and also as a linked in complex. You must calculate the whole demand for sodium thiosulfate, so the sum of moles from that two sources will be your answer.

BTW. In my post I found a mistake. Of course should be [Ag(S₂O₃)₂]^3- instead of [Ag(S₂O₃)]^3-