Question

A compound contains $N=25.94\%$ and $O=74.06\%$. If its vapour density is $54.2$, what is the molecular formula?

• A. $NO$
• B. $N{O}_2$
• C. $N_{2}O$
• D. $N_2{O}_5$

Answer 1

The correct option is D $N_2{O}_5$

A compound contains $N=25.94\%$ and $O=74.06\%$.

100 g of compound will contain 25.94 g of N and 74.06 g of O.

The atomic masses of N and O are 14 g/mol and 16 g/mol respectively.

The number of moles of $N=\frac{25.94g}{14g/mol}=1.85mol$.

The number of moles of $O=\frac{74.06g}{16g/mol}=4.63mol$.

The mole ratio $N:O=1.85:4.63=2:5$

Hence, the empirical formula (and the molecular formula) is $N_2{O}_5$

Note: Options A, B and C can be ruled out as they do not satisfy the mole ratio $N:O=2:5$.