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Q.
When $Cu ^{2+}$ ion is treated with $KI$, a white precipitate, $X$ appears in solution. The solution is titrated with sodium thiosulphate, the compound $Y$ is formed. $X$ and $Y$ respectively are
$Cu ^{2+}+2 KI \longrightarrow \underset{\text { Unstable }}{ CuI _2} \downarrow+2 K ^{+}$
$I ^{-}$is strong R.A it reduces $Cu ^{2+}$ to $Cu ^{+}$
$2 CuI _2 \longrightarrow \underset{\text { (White)'X' }}{ Cu _2 I _2 \downarrow}+ I _2$
$KI + I _2 \longrightarrow K ^{+} I _3^{-} \text {(Brown solution) }$
$I _3^{-} \rightleftharpoons I _2+ I ^{-}$
$KI _3+ Na _2 S _2 O _3 \rightarrow KI + \underset{(Y)}{Na _2 S _4 O _6}$