Redox titration calculation OCR

1.015g of potassium iodate was dissolves in water and made up to 250cm3 of solution. 25cm3 of this was added to excess KI solution in acid before being titrated against sodium thiosulphate. 29.80cm3 of this thiosulphate solution was required. Calculate conc of sodium thiosulphate.

Equations given - IO3- + 5I- + 6H+ > 3I2 + 3H2O
2S2O32- + I2 > S4O62- + 2I-

What I've done so far :
1.015/174.9 to get moles which is 5.8x10-3
I then divide this by 10 to get moles in 25cm3 which gives me 5.8x10-4
I then multiply this answer by 3 to get moles of Iodine
Then multiply it by 2 to get moles of S2O3 in the second equation
I then use the calculation conc = mol/vol

Can anyone explain where I'm going wrong? Thank you :smile:

)

(edited 7 years ago)

Reply 1

Eloades11

I've moved this to the chemistry study help forum where hopefully you'll get more responses :smile:

Reply 2

ftsoft

Original post by AqsaMx

1.015g of potassium iodate was dissolves in water and made up to 250cm3 of solution. 25cm3 of this was added to excess KI solution in acid before being titrated against sodium thiosulphate. 29.80cm3 of this thiosulphate solution was required. Calculate conc of sodium thiosulphate.

Equations given - IO3- + 5I- + 6H+ > 3I2 + 3H2O
2S2O32- + I2 > S4O62- + 2I-

What I've done so far :
1.015/174.9 to get moles which is 5.8x10-3
I then divide this by 10 to get moles in 25cm3 which gives me 5.8x10-4
I then multiply this answer by 3 to get moles of Iodine
Then divide it by 2 to get moles of S2O3 in the second equation
I then use the calculation conc = mol/vol

Can anyone explain where I'm going wrong? Thank you :smile:

)

Whats the answer???

Reply 3

pineneedles

Original post by AqsaMx

1.015g of potassium iodate was dissolves in water and made up to 250cm3 of solution. 25cm3 of this was added to excess KI solution in acid before being titrated against sodium thiosulphate. 29.80cm3 of this thiosulphate solution was required. Calculate conc of sodium thiosulphate.

Equations given - IO3- + 5I- + 6H+ > 3I2 + 3H2O
2S2O32- + I2 > S4O62- + 2I-

What I've done so far :
1.015/174.9 to get moles which is 5.8x10-3
I then divide this by 10 to get moles in 25cm3 which gives me 5.8x10-4
I then multiply this answer by 3 to get moles of Iodine
Then divide it by 2 to get moles of S2O3 in the second equation
I then use the calculation conc = mol/vol

Can anyone explain where I'm going wrong? Thank you :smile:

)

The ratio of thiosulfate to iodine is 2:1 so shouldn't you double the moles of iodine to find the moles of thiosulfate rather than dividing?

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Reply 4

AqsaMx

Original post by pineneedles

The ratio of thiosulfate to iodine is 2:1 so shouldn't you double the moles of iodine to find the moles of thiosulfate rather than dividing?

Posted from TSR Mobile

I miswrote that, I am multiplying it by 2 but I'm still not getting 0.095mol fm-3

Reply 5

AqsaMx

Original post by ftsoft

Whats the answer???

0.095mol fm-3

Reply 6

pineneedles

Original post by AqsaMx

I miswrote that, I am multiplying it by 2 but I'm still not getting 0.095mol fm-3

Your Mr at the beginning is wrong. The molar mass of potassium iodate is 214. Everything else is correct though, change that and you'll get the right answer.

Posted from TSR Mobile

Reply 7

AqsaMx

Original post by pineneedles

Your Mr at the beginning is wrong. The molar mass of potassium iodate is 214. Everything else is correct though, change that and you'll get the right answer.

Posted from TSR Mobile