Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: twomblyhero on June 18, 2014, 05:45:03 AM

We completed a redox titration of Iodine (25cm3)
(formed from 1M Potassium Iodide (KI), 0.1M Potassium Iodate (KIO_{3}) and excess Hydrochloric acid (KCl)
against Sodium thiosufate (Nas_{2}O_{3})
It took 25.8cm^{3} of 0.1 M Nas_{2}O_{3} 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
2S_{2}O_{3}^{2} (aq)+ I_{2} (aq) :rarrow: 2I (aq) + S_{4}O_{6}^{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.

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.

Thanks for your answer; i'm still unsure of what to do.
I know
5KI + KIO_{3} + 6HCL :rarrow: 3H_{2}O + 6KCl + 3I_{2}
and the ratio of 2S_{2}O_{3}^{2} (aq)+ I_{2} (aq) is 2:1 I can work out the concentration of I_{2} but then how do I work the rest out?
Any ideas folks?

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.

So 5 moles of Iodide ions (from KI) will react with 1 mole of Iodate ions (from KIO_{3})?
concentration= moles = 5 x 25/1000 = 0.0002mols dm^{3}
_____
volume
....still stuck!

5 mol iodide create 3 mole iodine.
2 mol thiosulfate correspond to 1 mol iodine
how many mole iodide correspond to thiosulfate?

1?

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.

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