Using mole ratio to determine concentration of a reactant?

Question

S₂O₈^2- + 2 I^- --> 2 SO₄^2- + I₂   (reaction 1)       [I^-]=0.20M, [S₂O₈^2-]=0.10M 

 

 

2 S₂O₃^2- + I₂ --> S₄O₆^2- + 2 I^-  (reaction 2)      [S₂O₃^2-]= 0.0050M

 

I need to determine the concentration of the I₂ in reaction 2, can I use the mole ratio to find it like this:

0.0050M S₂O₃^2- x 1mol I₂/2 mol S₂O₃^2- = 0.0025M I₂

 

 

Also, when given a concentration of a stock solution, the concentration given is the concentration of that reactant unless it is diluted, right? Or am I missing something?...

 

Answer 1

If the I₂ in reaction 2 is that which is generated in reaction 1, then you CANNOT do it the way you did it. You have to find the I₂ generated which would be 0.1 M. To answer the second question, yes it is the concentration of the stated ingredient.