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Chemistry Practical Questions

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Ykov:
 * Please explain, in detail, what is happening during the reaction(s), specifically focusing on:
    * Why is iodine still present in the solution during titration with sodium thiosulphate? Basically, the color gets paler as the reaction proceeds so you need an indicator to get an accurate value for the exact titre needed. The reaction was not yet completed when the starch indicator was added so Iodine is still present, even in minute amounts. The exact time the blue-black colour disappears = the exact time the reaction is completed
    * Wouldn't the solution turn brown yellow even if we do not titrate, because of the reaction with chromate in an acidic medium?
10 cm3 of dilute sulphuric acid and 10 cm3 of 5% aqueous potassium iodide, KI.

When you add KI, the Cromium has been reduced to Cr3+, leading to the pale blue colour mentioned, masked by the brown colour of the iodine solution
    * Doesn't the addition of starch and hence formation of a starch-iodine complex, hamper the reaction in any way? 

This possibly has to do with substrate-product chemistry, so I'm guessing the Thiosulphate ions displace the starch and reacts with Iodine

Chingoo:
Thank you again Ykov! I'm really greatly, and can't double rep but I would if I could =D

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