Question

What is the molecular weight of air?

Answer 1

We need to know that the atmosphere of Earth is the layer of gases, commonly known as air, retained by Earth's gravity, surrounding the planet Earth and forming its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation.

Complete answer:
Before solving this question we must know what is air composed of? Air is a mixture of several gases, where the two most dominant components in dry air are $21$ volume percentage oxygen and $78$ volume percentage nitrogen, Argon: $0.934$ percentage and Carbon dioxide: $0.03$ percentages.
Since both of these elements are diatomic in air – oxygen and nitrogen, the molar mass of gases are given by
The atomic / molecular weights are:
Nitrogen: $14.0067 \times 2$ = $28.0134g/mol$ (since nitrogen gas requires two atoms of nitrogen)
Oxygen: $15.9994 \times 2$ = $31.9988g/mol$ (since oxygen gas requires two atoms of oxygen)
Argon: $39.948g/mol$
Carbon dioxide: $44.01g/mol$
Weight of each gas by knowing the percentage
Nitrogen: $\dfrac{{78.084}}{{100}} \times 28.0134 = 21.8739g/mol$
Oxygen: $\dfrac{{20.964}}{{100}} \times 31.9988 = 6.7025g/mol$
Argon: $\dfrac{{0.934}}{{100}} \times 39.948 = {\text{0}}.373g/mol$
Carbon dioxide: $\dfrac{{0.03}}{{100}} \times 44.01 = 0.013203g/mol$
Add all the values we get,
$21.8739 + 6.7025 + 0.373 + 0.013203 = 28.96g/mol$. This is the molecular weight of the air.

Note:
We need to know that air composition, temperature, and atmospheric pressure vary with altitude, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres. The remaining gases are often referred to as trace gases, among which are the greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone.