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  4. A solution of sodium thiosulphate on addition of few drops of ferric chloride gives violet colour due to the formation of

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Q. A solution of sodium thiosulphate on addition of few drops of ferric chloride gives violet colour due to the formation of

UPSEEUPSEE 2008

Solution:

When a few drops of $FeCl_{3}$ solution is added to hypo solution, a violet colour of ferric thiosulphate is obtained
$3Na_{2}S_{2}O_{3} + 2FeCl_{3} {\rightleftharpoons} \underset{\text{violet colour}}{{ Fe_{2}\left(S_{2}O_{3}\right)_{3}}}+6NaCl$
This colour disappears quickly due to reduction of ferric chloride by thiosulphate
$2FeCl_{3} +2Na_{2}S_{2}O_{3} \to 2FeCl_{2} + Na_{2}S_{4}O_{6}+2Nacl$