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Stoichiometric Calculations

5 g sample of...

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 $K I$ , 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 $C u$ in the alloy.

A

41.5 %

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B

20.5 %

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C

14 %

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D

None of these

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Solution

The correct option is A $41.5 %$
$2 C u^{2 +} + 2 I^{-} ⇌ 2 C u^{+} + I_{2}$
$2 N a_{2} S_{2} O_{3} + I_{2} ⇌ N a_{2} S_{4} O_{6} + 2 N a I$
Thus, $1 m o l C u^{2 +} = 1 m o l N a_{2} S_{2} O_{3}$
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 $= \frac{2.076}{5} \times 100 = 41.5$ %.

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