how would i work this out?

Question

I have done a redox titration with sodium thiosulphate and copper sulphate to find the concentration of copper ions. I used potassium iodide. I know the reactions that happen, but I can't figure out how to use the info that I have (average titre, conc., no. of moles, etc) to work out what the concentration of copper ions is.

Answer 1

First, you need to write balanced equations for the reactions. Then if x is the original level of copper ion in moles/cc, V, the original volume of CuSO4 used, xV is the number of moles of copper. The equation tells you the moles of thiosulfate needed to react with 1 mole of copper. If this is Q, and Vt, is the volume of titrant, and s, the conc of thiosulfate, then xV= QsVt. I believe the KI is used as an indicator of the reaction being completed, at which point iodine, with its purple color forms.

Answer 2

If you follow I2 through the equations, you will see that the number of moles of Cu2+ and thiosulphate are equal - a 1:1 ratio.

So you can do a normal titration calculation, despite there being two equations.