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hard titration question help

can someone explain the working out for the answer which is 237.17 micrograms
(edited 3 years ago)
Screenshot 2021-04-13 194340.jpg284.7KB
Original post by sara.m12
can someone explain the working out for the answer which is 237.17 micrograms

It is a back titration- find moles I2 at the start and at the end. The amount of I2 used up will give you the amount of SO2 in the air sample:
I2.jpg
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sara.m12
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Thank you, that really helped. May I ask how you know when the question is a back titration? I do edexcel a level and it isnt in my spec however this was a past paper question. Are these common qs?

also, regarding the question, how did you know that the iodine and thiosulphate ratio was 1:2?

Thank you for your help!!
Original post by Davies Chemistry
It is a back titration- find moles I2 at the start and at the end. The amount of I2 used up will give you the amount of SO2 in the air sample:
I2.jpg
Original post by sara.m12
Thank you, that really helped. May I ask how you know when the question is a back titration? I do edexcel a level and it isnt in my spec however this was a past paper question. Are these common qs?

also, regarding the question, how did you know that the iodine and thiosulphate ratio was 1:2?

Thank you for your help!!

A back titration is when you start of with a known amount of something (iodine here) and then add something to react with it. In the titration you find out how much (in this case of the iodine) is left over.

You know the I2 to thiosulphate (S2O32-) ratio as it is given in the equation at the top of the question.
thio.jpg

Edexcel are very keen on iodine /thiosulphate titrations so it is as well to know this equation off by heart.
Reply 4
Avatar for sara.m12
sara.m12
OP
16
Original post by Davies Chemistry
A back titration is when you start of with a known amount of something (iodine here) and then add something to react with it. In the titration you find out how much (in this case of the iodine) is left over.

You know the I2 to thiosulphate (S2O32-) ratio as it is given in the equation at the top of the question.
thio.jpg

Edexcel are very keen on iodine /thiosulphate titrations so it is as well to know this equation off by heart.


Thank u!!

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