Question

A solution of $0.2g$ of a compound containing $C{u}^2+$ and $C_2{O}_4^{2-}$ ions on titration with $0.02MKMn{O}_4$ in presence of $H_{2}S{O}_4$ consumes $22.6mL$ of the oxidant. The resultant solution is neutralised with $N{a}_{2}C{O}_3$, acidified with dilute acetic acid and treated with excess $KI$. The liberated iodine requires $11.3mL$ of $0.05MNa$ ${}_2{S}_2{O}_3$, solution for complete reduction. Find out the mole ratio of $Cu-+$ to $C_2{O}_4^{2-}$ in the compound. Write down the balanced redox reactions involved in the above titrations.

Answer 1

With $KMn{O}_4$, oxalate ion is oxidised only as :
$5C_2{O}_4^{2-}+2Mn{O}_4^{-}+16H^{+}⟶2M{n}^{2+}+10C{O}_2+8H_{2}O$
Let, in the given mass of compound, $x$ millimol of $C_2{O}_4^{2-}$ ion is present, then
Meq of $C_2{O}_4^{2-}=$ Meq of $Mn{O}_4^{-}$
$\Rightarrow 2x=0.02\times 5\times 22.6$
$\Rightarrow x=1.13$
At the later stage, with $I^{-},C{u}^{2+}$ is reduced as :
$2C{u}^{2+}+4I^{-}⟶2CuI+I_2$
and $I_2+2S_2{O}_3^{2-}⟶2I^{-}+S_4{O}_6^{2-}$
Let there be $x$ millimol of $C{u}^{2+}$.
$\Rightarrow MeqofC{u}^{2+}=Meq$ of $I_2=$ meq of hypo
$\Rightarrow x=11.3\times 0.05=0.565$
$\Rightarrow$ Moles of $C{u}^{2+}:$ moles of $C_2{O}_4^{2-}=0.565:1.13=1:2$