Question

Fill in the blanks
Molecular mass of dry air is_____________
A.Less than moist air
B.Greater than moist air
C.Equal to moist air
D.Maybe greater or less than moist air

Answer 1

We must remember that in the earth’s atmosphere there are various types of gases liberated in the air coming from various sources like industries, burning, oil stations, etc. The air in the atmosphere consists of nitrogen and oxygen along with water molecules. The amount of water molecules present in the air varies under different temperature and weather conditions.

Complete step by step answer:
We must know that the dry air consists of nitrogen and oxygen. Hence, if we calculate the molecular mass then it will be,
${N_2} = 28gm$
${O_2} = 32gm$
Total molecular mass = $60gm$
While the moist air contains only water vapors and no other atoms or molecules. If we calculate the molecular mass then it will be,
${H_2} = 2gm$
$O = 16gm$
${\text{Total = }}18gm$
Hence, the molecular mass of moist air is less in comparison with dry air.
Also, if we want to make the dry air moist, we need to add water vapors in the atmosphere. If we do so the water vapors will remain in the air as moist air and the water constituents will be replaced by nitrogen and oxygen which are contained in dry air.
The correct answer to the question is option B, greater than moist air.

Note:
We have to know that the water molecules present in the moist air contributes to the humidity of the air. The dry air is ideally composed of nitrogen and oxygen which covers the $99\%$ of the earth’s atmosphere. We have many times heard a hypothesis which says that – “Water is heavier than air”. It is correct that water in the liquid form is heavier than air due to its high density of molecules. But if we talk about air in the atmosphere the water is present in vapor form and air can move freely in the atmosphere due to the huge amount of free space.