Question

Air contains $N_2$ and $O_2$ in $4:1$ ratio by volume. Approximate molecular weight of air (g/mole) is:

• A. $29$
• B. $38$
• C. $16$
• D. $74$

Answer 1

The correct option is A $29$

The molecular weight of air $=$ mole fraction of nitrogen $\times$ molecular weight of nitrogen $+$ mole fraction of oxygen $\times$ molecular weight of oxygen.

Now, mole fraction of nitrogen $=\frac{4}{5}=0.80$ and mole fraction of oxygen $=\frac{1}{5}=0.20$.

So, molecular weight of air $=0.8\times 28+0.2\times 32=29$ g/mole.

Hence, the correct answer is option $\text{A}$.