[twomblyhero]

We completed a redox titration of Iodine (25cm3)
(formed from 1M Potassium Iodide (KI), 0.1M Potassium Iodate (KIO₃) and excess Hydrochloric acid (KCl)


against Sodium thiosufate (Nas₂O₃)

It took 25.8cm³ of 0.1 M Nas₂O₃ to get rid of the formed Iodine.

The question asks:

Use the equation below to calculate the amount of iodide ions that reacts with each mole of iodate ions in aqueous solution

2S₂O₃^2- (aq)+ I₂ (aq)     2I- (aq) + S₄O₆^2-(aq)

and determine the concentration of iodide ions in moldm^-3

I am not sure what this is asking me! Do I need to calculate how the Iodine is originally formed?
I am really stuck on this one! Please help.

[Hunter2]

They want to know how much iodide was in the solution.

You have a sample of Iodide it reacts with iodate to iodine. Develop here the equation as well. The iodine is titrated. So you can calculate back.

[twomblyhero]

Thanks for your answer; i'm still unsure of what to do.

I know
5KI + KIO₃ + 6HCL  3H₂O + 6KCl + 3I₂

and the ratio of 2S₂O₃^2- (aq)+ I₂ (aq) is 2:1 I can work out the concentration of I₂ but then how do I work the rest out?

Any ideas folks?

[Hunter2]

Simple mathematics. You see how much iodide creates iodine.  And youn see how much thiosulfate is used for Iodine. You have to calculate the relation Iodide Thiosulfate.

[twomblyhero]

So 5 moles of Iodide ions (from KI) will react with 1 mole of Iodate ions (from KIO₃)?

concentration= moles         =  5   x 25/1000 = 0.0002mols dm^-3
                     _____         
                      volume       

....still stuck!

[Hunter2]

5 mol iodide create 3 mole iodine.

2 mol  thiosulfate correspond to 1 mol iodine

how many mole iodide correspond to thiosulfate?

[twomblyhero]

1?

[Hunter2]

No it is 6 mol Thiosulfate correpond to 5 mol Iodide.

With his knowledge calculate how many mole 25.8 ml of 0,1 M thiosulfate is.  If you have this then you can calculate the content of iodide. This you have to get in relation to your 25 ml sample.

[twomblyhero]

Thanks for you help, I understand the ratios now. Just got to do the calculations now.