Question

What is the molecular weight of air?

Answer 1

Step1. Molecular weight

• Molecular weight is the sum of atomic weight, measured in amu.
• The mass of one carbon atom is equal to the atomic mass unit (formerly abbreviated as 'amu,' but now spelled as 'u' – unified mass) according to the latest IUPAC recommendation.

Step2. The formula for molecular weight of air

• Air is a mixture of several gases.
• The two most dominant components in dry air are nitrogen $\left({\mathrm{N}}_2\right)$and oxygen $\left({\mathrm{O}}_2\right)$. Traces of carbon dioxide$\left({\mathrm{CO}}_2\right)$ and argon$\left(\text{Ar}\right)$ are also present in the air.
• The proportion of nitrogen is $78\%$, oxygen is $21\%$, carbon dioxide is $0.03\%$, and argon is $0.934\%$
• So, the formula for the molecular weight of air is as follows:-

The molecular weight of air =$\left[\left(2\times \text{Atomic weight of}{\mathrm{N}}_2\text{×0.78}\right)\text{+}\left(\text{2×}\mathrm{Atomic}\mathrm{weight}\mathrm{of}{\mathrm{O}}_2\text{×0.21}\right)\text{+}\left(\text{Atomic weight of C+Atomic weight of O×2}\right)\text{0.0003+Atomic weight of Ar×0.00934}\right]$

Step3. Calculation of molecular weight of air

The molecular weight of air =$\left[\left(2\times 14.0067\text{×0.78}\right)\text{+}\left(\text{2×16×0.21}\right)\text{+}\left(\text{12.01+16×2}\right)\text{0.0003+39.95×0.00934}\right]\text{u}$

=$\left[\text{21.85+6.72+0.013203+0.373133}\right]\text{u}$

=$28.956\text{u}$

$\simeq 28.96\text{u}$

Hence, the molecular weight of air is $28.96\text{u}$.