Question

10g of $$\mathrm{MnO}_{2}$$ on reaction with HCl forms 2.24 L of $$\mathrm{Cl}_{2}$$(g) at NTP, the percentage impurity of $$\mathrm{MnO}_{2}$$ is:
A) 87%
B) 25%
C) 33.3%
D) 13%

Answer 1

Hint: Initially, from the balanced chemical equation we will find out the number of moles in STP. After a complete reaction with the chlorine present , the rest manganese from the given 10g will give us the impurity.

Complete step by step answer:
Stoichiometry is the quantitative relationships between the reactants and products formed. The term has been derived from the greek words- “stoicheion” meaning element and “metron” meaning measurement. Thus it is important for us to have a balanced chemical equation.
The Law of conservation of mass dictates that the quantity of each element does not change in a chemical reaction. Thus, each side of the chemical equation must represent the same quantity of any particular element. Likewise, the charge is conserved in a chemical reaction. Therefore, the same charge must be present on both sides of the balanced equation.
Now, the balanced chemical reaction involved in the question is:
$$\mathrm{MnO}_{2}$$ +4HCl→$$\mathrm{MnCl}_{2}$$ + $$\mathrm{Cl}_{2}$$ +$$2 \mathrm{H}_{2} \mathrm{O}
$$

Here, as per the given equation, 1 mole of $$\mathrm{Cl}_{2}$$ is formed from 1 mole of $$\mathrm{MnO}_{2}$$
Therefore, Number of moles of $$\mathrm{Cl}_{2}$$ is n= 2.24L/22.4L= 0.1 mol
 Therefore, Mass of $$\mathrm{Cl}_{2}$$ is W= 87g/mol × 0.1 mol = 8.7 g
 Impurity in $$\mathrm{MnO}_{2}$$ = 10g−8.7g = 1.3g
Now, %age impurity of $$\mathrm{MnO}_{2}$$ = (1.3g/10g) X 100
                                                = 13%
So, the correct answer is “Option D”.

Note: Always calculate these stoichiometric problems after balancing the chemical equation. In case it is not balanced, we should balance first and then proceed with the calculations.