2014 jan unit 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?

Reply 2

OP

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??

Reply 3

Can you work out the equation for the reaction between iodine pentoxide and carbon monoxide?

Reply 4

OP

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 :frown:

btw are all iodine thiosulfate titrations back titration?

(edited 8 years ago)

Reply 5

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

Reply 6

OP

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??

Reply 7

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.

Reply 8

OP