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Sulphonation of Aromatics

5 g of pyrolu...

Question

5 g of pyrolusite (impure $M n O_{2}$ ) was heated with conc. $H C l$ and $C l_{2}$ evolved was passed through excess of $K I$ solution. The iodine liberated required 40 mL of $\frac{N}{10}$ hypo solution. Find the $%$ of $M n O_{2}$ in the pyrolusite (atomic mas of Mn = 55 amu).

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Solution

$M n O_{2} + 4 H C l \to M n C l_{2} + C l_{2} + 2 H_{2} O$

$C l_{2} + 2 K I \to 2 K C l + I_{2}$ and
$I_{2} + 2 N a_{2} S_{2} O_{3} \to 2 N a I + N a_{2} S_{4} O_{6}$

40 mL of $\frac{N}{10}$ hypo solution = 4 meqv of $N a_{2} S_{2} O_{3}$ = 4 meqv of $I_{2}$

= 4 meqv $C l_{2}$ (n-fac is same i.e. 2 for $I_{2}$ in both reaction)

= 4 meqv of $M n O_{2}$ (n-fac is same i.e. 2 for $C l_{2}$ in both reaction)

= 2 mmol of $M n O_{2}$ (n-fac is 2)

= $2 m m o l \times 87$ g of $M n O_{2}$ (molar mass 87 g/mol)

= 0.174 g

= $\frac{0.174}{5} \times 100 = 3.48 %$ of $M n O_{2}$

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