[Borek]

I₂ + 2 Na₂S₂O₃ ? 2 Na₂S₄O₆ + 2 I^-

Huh?

Besides, I am not sure what you did.

You should use substantial excess of iodide, and you should calculate amount of iodide based on expected amount of chlorine you will be analyzing. AFAICNT you did the opposite - you have started with KI.

Final result in case of gases can be safely presented as v/v percentage (no volume changes during mixing) and it will be more accurate then you did - you have assumed mixture has density of pure Cl₂, which for obvious reasons is 10% not true

[Alberto_Kravina]

Sorry Borek! stupid typing mistake... of course it is
I2 + 2 Na2S2O3 ?  Na2S4O6 + 2 NaI

[Alberto_Kravina]

Ok, I'll try to explain what I did:

I estimated a volume of Chlorine for 20 mL of my KI solution (0,1n). I used an excess iodide to be sure that 100% Cl₂ reacts with I^-. (I should have used about 11,2 mL of Chlorine, but I used only 10mL...so there was an excess I^-. The chlorine gas didn't bubble out, by the way...

Final result in case of gases can be safely presented as v/v percentage (no volume changes during mixing) and it will be more accurate then you did

true!

Is it clear now?

[Borek]

I estimated a volume of Chlorine for 20 mL of my KI solution (0,1n). I used an excess iodide to be sure that 100% Cl₂ reacts with I^-. (I should have used about 11,2 mL of Chlorine, but I used only 10mL...so there was an excess I^-. The chlorine gas didn't bubble out, by the way...

I would go for much higher excess. Note that iodine is weakly soluble in water, it is reasonably soluble in iodide due to I₃^- formation - so you need at least 150% iodide to be able to convert all iodine to I₃^-. I would go for 400% excess or something like that - it won't hurt. No idea what is formation constant for I₃^-, this may give some indication.

As for direction of calculations (whether to start with expected amount of chlorine in sample to calculate volume of iodide needed, or assume some volume of iodide solution to check what volume of chlorine can be analysed) - this is much more subtle. For me obvious way will be to start with 10 mL of chlorine to check how it translates to amount of iodide needed (ie starting calculation from analyte side) and than adjust procedure (ie iodide solution amount) - reducing sample volume if needed, but your approach has its advantages too. Let's call it a draw

As for reaction - go for net ionic in such situations.

[Alberto_Kravina]

Yeah, maybe I should have used more KI...but my result seems to be pretty plausible...