Question

Dry air has a molar composition of $22\%O_2$ and $78\%N_2$. Calculate the average molecular weight of dry air.

• A. $28.4\text{g/mol}$
• B. $25.8\text{g/mol}$
• C. $28.9\text{g/mol}$
• D. $27.9\text{g/mol}$

Answer 1

The correct option is C $28.9\text{g/mol}$

The molecular mass of $O_2$ and $N_2$ are $32\text{g/mol}$ and $28\text{g/mol}$ respectively.
Average molecular weight $=(32\times 0.22)+(28\times 0.78)$
$=28.88\text{g/mol}$ or 28.9 g/mol