Question

What is the molecular weight of air?

Answer 1

Hint :The mass of a molecule is measured in daltons and is called molecular mass. Since they contain different isotopes of an element, different molecules of the same compound can have different molecular masses.

Complete Step By Step Answer:
The Earth's atmosphere is a layer of gases, commonly known as air, that surrounds the planet Earth and forms its planetary atmosphere, and is held in place by gravity. Dry air contains $78.084\% $ nitrogen, $20.946\% $ oxygen, $0.934\% $ argon, $0.03\% $ carbon dioxide, and trace quantities of other gases by volume.
In order to find the concentration of all these four air, we will have to add the concentration of all these four air - $78.084\% $ $ + $ $20.946\% $ $ + $ $0.934\% $ $ + $ $0.03\% $ $ = $ $99.994\% $
The concentration of remaining gases in the air $ = (100 - 99.994)\% $
$ = 0.006\% $
These figures are volume percentages, and one mole of gas occupies $22.4$ litres at room temperature and pressure (but that is really irrelevant to this question as the volumes cancel).
In order to find the molecular weight of the dry air, firstly we have to find the molecular weight of the gases (as a molecule).
We know that the atomic weight of a Nitrogen atom is $14.0067$.
Since the Nitrogen air is made up of two atoms of Nitrogen, it is written as ${N_2}$.
The molecular weight of Nitrogen air $ = 14.0067 \times 2$$ = 28.0134{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$
We know that the atomic weight of an Oxygen atom is $15.994$.
Since the Oxygen molecule is also made up of two atoms of Oxygen, it is written as ${O_2}$.
The molecular weight of Oxygen air $ = 15.994 \times 2$$ = 31.9988{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$.
The molecular weight of Argon air $ = 39.948{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$
The molecular weight of Carbon Dioxide $ = 41.01{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$
Since in the above section, we have discussed about the percentage of composition of each air, now we can find out the weight of each air in the dry air –
Nitrogen: $({{78.084} \mathord{\left/
 {\vphantom {{78.084} {100) \times 28.0134 = 21.8739{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}}}} \right.} {100) \times 28.0134 = 21.8739{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}}}$
Oxygen: $({{20.946} \mathord{\left/
 {\vphantom {{20.946} {100)}}} \right.} {100)}} \times 31.9988 = 6.7025{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$
Argon: $({{0.934} \mathord{\left/
 {\vphantom {{0.934} {100)}}} \right.} {100)}} \times 39.948 = 0.373{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$
Carbon Dioxide: $({{0.03} \mathord{\left/
 {\vphantom {{0.03} {100) \times 44.01 = 0.013203{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}}}} \right.} {100) \times 44.01 = 0.013203{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}}}$
Now, on adding all the weight, we get the total molecular weight of the dry air $ = 21.8739 + 6.7025 + 0.373 + 0.013203 = 28.96{g \mathord{\left/
 {\vphantom {g {mol}}} \right.} {mol}}$.

Note :
Air composition, temperature, and ambient pressure differ with altitude, and only the Earth's troposphere and artificial atmospheres provide air suitable for photosynthesis by terrestrial plants and breathing by terrestrial animals.