Question

$5$ g sample of brass was dissolved in one litre dil.$H_{2}S{O}_4$. $20$ mL of this solution were mixed with $KI$, liberating $I_2$ and $C{u}^{+}$ and the $I_2$ required $20$ mL of $0.0327$ N hypo solution for complete titration. Calculate the percentage of $Cu$ in the alloy.

• A. $41.5\%$
• B. $20.5\%$
• C. $14\%$
• D. None of these

Answer 1

The correct option is A $41.5\%$

$2C{u}^{2+}+2I^{-}\rightleftharpoons 2C{u}^{+}+I_2$
$2N{a}_2{S}_2{O}_3+I_2\rightleftharpoons N{a}_2{S}_4{O}_6+2NaI$
Thus, $1molC{u}^{2+}=1molN{a}_2{S}_2{O}_3$
Hence, total number of moles of copper in 20 ml ofsample $=\frac{20\times 0.0327}{1000}=6.54\times {10}^{-4}$ mol.
Hence, total number of moles of copper in 1000 ml of sample $=\frac{1000\times 6.54\times {10}^{-4}}{20}=0.0327$ mol.
Hence mass of copper $=63.5\times 0.0327=2.076$ g
Percentage of copper in sample $=\frac{2.076}{5}\times 100=41.5$ %.