Question

$8.7$ g of pyrolusite (impure $Mn{O}_2$) was heated with conc. $HCl$. The $C{l}_2$ gas evolved was passed through excess of $KI$ solution. The iodine gas evolved required $80$ ml of $\frac{N}{10}$ hypo solution. The percentage of $Mn{O}_2$ in pyrolusite will be :

• A. $4\%$
• B. $40\%$
• C. $80\%$
• D. $8\%$

Answer 1

The correct option is A $4\%$

The reactions involved are as follows:

$Mn{O}_2+4HCl\to MnC{l}_2+C{l}_2+2H_{2}O$

$C{l}_2+2KI\to I_2+2KCl$

$I_2+2N{a}_2{S}_2{O}_3\to 2NaI+N{a}_2{S}_4{O}_6$
Meq. of $Mn{O}_2=$ Meq. of $C{l}_2=$ Meq. of hypo

$⟹$ Meq. of $Mn{O}_2=$ Meq. of hypo

$\frac{m}{87/2}\times {10}^3=\frac{1}{10}\times 80$
$⟹m=\frac{1}{10}\times 80\times \frac{87}{2}\times \frac{1}{{10}^3}$
$⟹m=\frac{4\times 87}{1000}g$

$\%$ purity of $Mn{O}_2=\frac{\frac{4}{1000}\times 87}{8.7}\times 100=4\%$.

Hence, option A is correct.