redox titrations

[chemkid_23]

Hi all,
Got a redox prac sac this week. WE've been given the half equations:
I2 + 2e- --> 2I-         and       2S2O32- --> S4O62- + 2e-

When they're brought together
I2 + 2S2O32- --> 2I- + S4O62-

The original balanced equation would be
I2 + 2Na2S2O3 --> 2NaI + Na2S4O6

My question is, which equation do we use when calculating mol ratios to finally find percentage by mass?

Na2S2O3 (20ml and 0.1M) is pipetted; I2 (0.05M) is titre

Thanks

[thushan]

You can use the second last or last equation.

[zool3]

they're the same, just one doesnt have the spectator ions.
depending on what the question ask you. if it ask for, eg. Na2S2O3, use the second but if it just 2S2O32- use the first. but either it should work out to be the same

[chemkid_23]

I see what you mean.
Thanks for your help