Question

The percentage by mole of $N{{O}_{2}}$ in a mixture of $N{{O}_{2}}(g)$ and $NO(g)$ having average molecular mass 34 is:
A. 25%
B. 20%
C. 40%
D. 75%

Answer 1

Hint: Mole concept gives a relationship between number of moles, weight and the molar mass of a compound. Molecular mass a molecule can be calculated by adding the atomic masses of all the individual elements which are present in the molecule.

Complete answer: - In the question it is given to calculate the percentage of $N{{O}_{2}}$ in a mixture of $N{{O}_{2}}$ and $NO$ .
- In order to obtain the percentage by mole of the given gas, let us consider it as ‘x’.
- Now, we know that the molecular mass of nitrogen dioxide is 46 g/mol and the molecular mass of nitric oxide is 30 g/mol.
- The average molecular mass can be determined by measuring the molecular mass of molecules and then adding the masses with later dividing by n.
- Thus, the average molecular mass is given as
\[46\times x+30\dfrac{100-x}{100}=34\]
- On solving the above equation, we will get x = 25 %.
- So, the average molecular mass of the $N{{O}_{2}}$ in a mixture of $N{{O}_{2}}$ and $NO$ is 25%.

Therefore, the correct option is (A).

Note: Mole is a scientific unit and can be defined as the ratio of weight and molecular weight of the atoms or molecules. It is defined as the amount of substance present in a sample. Molar mass or atomic mass is defined as the addition of individual atomic masses of atoms which are present in a molecule. It is calculated by adding the atomic mass of all the elements present in the compound.