Question

4.35 g of a sample of pyrolusite (MnO₂) when heated with conc. HCI gave chlorine. The chlorine when passed through potassium iodide solution liberated 6.35 g of iodine. The percentage purity of the pyrolusite sample is

a. 40

b. 50

c. 60

d. 70

Answer 1

Correct option is b. 50

Answer 2

Correct option is (b) 50

 \(MnO_2+4HCl\longrightarrow MnCl_2+Cl_2+2H_2O\)

Chlorine gas is passed through KI

\(Cl_2+2KI\longrightarrow 2KCl+I_2\)

Given \(I_2\) liberated \(=6.35g\)

Number of moles of \(I_2\) liberated \(=\frac{6.35}{254}=0.025\) mol.

from above equation 

1 mole \(MnO_2\) liberated = 1 mol \(Cl_2\)

and one mole \(Cl_2\) liberated = 1 mol \(I_2\)

it means, 0.025 mole \(I_2\) liberated by 0.025 mole of \(MnO_2\)

\(\therefore\) mass of \(MnO_2 = 0.025\times87=2.175g.\)

\(\therefore\) % purity \(=\frac{2.175}{4.35}\times100=50\)%.