Question

A gaseous compound of nitrogen and hydrogen contains 12.5 % hydrogen by mass. The molecular formula of the compound if its relative molecular mass is 32, is: [N=14, H=1]

• A. $N{H}_4$
• B. $N_2{H}_3$
• C. $N_2{H}_6$
• D. $N_2{H}_4$

Answer 1

The correct option is D $N_2{H}_4$

100 g of the compound contains 12.5 g H and $100-12.5=87.5$ g N.
The atomic masses of H and N are 1 g/mol and 14 g/mol respectively.
The number of moles of H $=\frac{\text{12.5 g}}{\text{1 g/mol}}=\text{12.5 mol}$
The number of moles of N $=\frac{\text{87.5 g}}{\text{14 g/mol}}=\text{6.25 mol}$
The mole ratio $N:H=6.25:12.5=1:2$
Hence, the empirical formula of the compound is $N{H}_2$.
Molecular formula $=\left(N{H}_2\right)\times \frac{\text{Molecular weight}}{\text{empirical formula weight}}$
Molecular formula $=\left(N{H}_2\right)\times \frac{\text{32 g/mol}}{\text{16 g/mol}}$
The molecular formula of the compound is $N_2{H}_4$.