[jessicarodgers]

Hi, this is for a chemistry lab and I'm really not sure how to do any of these questions. I would really appreciate some help because I am stumped.


In an experiment, 5.00 mL of 0.1440 mol L–1 aqueous KIO3 was reacted with an excess of aqueous iodide, and the molecular iodine, I2, formed was titrated against 0.1000 mol L–1 aqueous S2O3
 2–.
The equation for the titration reaction is: I2(aq) + 2 S2O32–(aq) --> S4O62–(aq) + 2 I–(aq)
 
The results of the titration are summarized in the table:


Concentration of S2O32–: 0.1000 mol L –1
Concentration of KIO3: 0.1440 mol L –1
Volume of KIO3 per titration: 5.00 mL
Concentration of HCl: 1.0 mol L –1
Volume of HCl per titration: 10.0 mL
Concentration of KI 0.5 mol L–1
Volume of KI per titration: 10.0 mL
Indicator: starch
Colour change at endpoint: blue --> colourless

Volume of Na2S2O3(aq) required: Burette Reading (mL)
                                  Trial 1     Trial 2     Trial 3
Final burette reading:   45.00     44.27     45.28
Initial burette reading:   0.52       1.22       2.33
Volume Na2S2O3 used: 44.48    43.05     42.95
 Average of the two volumes agreeing within 0.10 mL = ___43mL______

QUESTIONS:

1. Use the titration data supplied to determine the moles of iodine produced in the reaction between
IO3–, I– and H+ (above).

2. Determine the molar ratio of iodate ions to iodine molecules by dividing each mole quantity by the
lowest mole quantity. IO3– : I2.
 

3. Use this ratio write the balanced equation for the reaction between iodate and iodide ions.
 

4. Confirm your results by balancing the redox equation for this reaction, showing the oxidizing and
reducing half reactions as well as the net ionic equation.
 
 
5. What effect on the calculated iodine concentration would be observed if you prepared your three
reaction mixtures ahead of time and left them uncovered on your bench before titrating? Explain.

[Arkcon]

OK, lets begin to solve this, as required by the forum rules:  http://www.chemicalforums.com/index.php?topic=65859.0

Lets start with part 1:  You've been given a volume and a concentration of KIO3, how much iodine, I₂ is that?

You can also probably go ahead and do Part 4, you're supposed to use that to confirm other calculations, might as well have that done beforehand, and compare your experiment to that.

[jessicarodgers]

I calculated the moles of KIO3 and I got 7.2 X10^-4 mol.
So, does that mean I have that same amount of moles of I2?

[Arkcon]

That would depend on a balanced red-ox reaction for the reaction of iodate and iodide to iodine.  (Say that 5 times fast.)  A reaction which hasn't been given yet.