AS Chemistry help... Watch

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dfghj
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#1
Report Thread starter 13 years ago
#1
I'm a bit stuck on this exam question, can anyone help?

A commercial bleach contains sodium chlorate (I). A 10cm3 sample of this bleach was diluted to 250cm3 in a volumetric flask. When a 25cm3 portion of the solution produced was added to an excess of acidified potassium iodide, iodine was produced. On titration, this iodine reacted exactly with 23cm3 of 0.1M sodium thiosulphate.

i) Write an equation for the reaction of the chlorate ions with iodide ions in the presence of H+ ions and for the reaction of iodine with sodium thiosulphate and describe what you see at the end point of titration. Name a suitable indicator.

ii) Calculate the concentration of the sodium chlorate in the original soultion in mol dm^-3

(17 marks)
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Borek
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OCl- is a strong oxidizer, it oxidises I- to I2, thats the easy part.

I2 is an oxidizer strong enough to convert S2O3(2-) to S4O6(2-), that's what the iodometry is all about.

There is an organic compund that reacts with iodine giving blue color.

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Borek
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Mehh
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That organic compound is called starch. Its readily available from potatoes. You can use that as an indicator of iodine easily...although it is generally the OTHER way round. However iodine reacts with starch to form a complex, hence will affect the titration. Hence the starch should only be used very late in the titration when the iodine colour is almost completely gone.
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Borek
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I didn't post the name 'starch' in a vague hope that I will force Velouria to make some digging on her own

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Borek
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Mehh
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sorry :p:
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dfghj
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Oops sorry, I should have known that starch could be used as an indicator - but thanks anyway!

Can anyone check my answer for me, please?

i)

ClO- + 2H+ + 2I- ----> Cl- + H2O + I2

I2 + 2SO2O32- ----> S4O62-
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