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# 2014 jan unit 2 Watch

1. can someone pleaseeee explain this question ... why does it appear to me as if the titration has been reversed!! in all the other questions titrating the solution with thiosulfate was the second step but here it is the first im so confused. someone pls explain

can u also pls meantion the reactions
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2. Reaction 1 makes some I2

This I2 is added to an excess of S2O32-

The amount of unreacted S2O32- is found by titration (against known I2)

What's the problem?
3. (Original post by Pigster)
Reaction 1 makes some I2

This I2 is added to an excess of S2O32-

The amount of unreacted S2O32- is found by titration (against known I2)

What's the problem?
arent we supposed to be finding the concen. of the oxidising agent??
4. Can you work out the equation for the reaction between iodine pentoxide and carbon monoxide?
5. (Original post by Pigster)
Can you work out the equation for the reaction between iodine pentoxide and carbon monoxide?
I2O5 + CO ---> I2 (want we want is I2) so the oxidising agent here was I2O5 .. isnt thiosulfate supposed to be a standard solution here.. why is it I2 !!!!!! this question is so confusing
btw are all iodine thiosulfate titrations back titration?
6. You haven't balanced the equation.

Things you know (can work out):

How many mol of I2 in the last titration.
How many mol of S2O32- that reacted with that I2
How many mol of S2O32- there were initially
How many mol of those S2O32- must have reacted with the I2 (from I2O5), i.e. the rest
How many mol of I2 that reacted with that S2O32-
How many mol of I2O5 there must have been to make that I2
How many mol of CO must have reacted with the I2O5 to make those I2
7. (Original post by Pigster)
You haven't balanced the equation.

Things you know (can work out):

How many mol of I2 in the last titration.
How many mol of S2O32- that reacted with that I2
How many mol of S2O32- there were initially
How many mol of those S2O32- must have reacted with the I2 (from I2O5), i.e. the rest
How many mol of I2 that reacted with that S2O32-
How many mol of I2O5 there must have been to make that I2
How many mol of CO must have reacted with the I2O5 to make those I2
thanks!! - this made it so much clearer x
but in titrations isnt sodium thiosulfate always supposed to be the standard solution??
8. In the real world, we typically use S2O32- as a standard as it can be made up to whatever conc you need and it is stable, i.e. it doesn't tend to decompose in solution.

You can make up a standard solution of iodine - you just dissolve whatever mass of I2 in whatever volume you need - but I wouldn't be as happy leaving it on the shelf as S2O32-.

I guess, in the case of this Q, the person who wrote it just wanted to make harder by adding the extra step.
9. (Original post by Pigster)
In the real world, we typically use S2O32- as a standard as it can be made up to whatever conc you need and it is stable, i.e. it doesn't tend to decompose in solution.

You can make up a standard solution of iodine - you just dissolve whatever mass of I2 in whatever volume you need - but I wouldn't be as happy leaving it on the shelf as S2O32-.

I guess, in the case of this Q, the person who wrote it just wanted to make harder by adding the extra step.
aha ok - thanks x

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