Question

Which are heavier than dry air?
A) Moist air
B) ${\text{S}}{{\text{O}}_{\text{2}}}$
C) ${\text{C}}{{\text{l}}_{\text{2}}}$
D) ${{\text{O}}_{\text{2}}}$

Answer 1

Hint:To solve this we must know that dry air is the air which does not contain any moisture or water molecules. The molar mass of dry air is about $28.97{\text{ g/mol}}$. To solve this we must know the molar masses of moist air, ${\text{S}}{{\text{O}}_{\text{2}}}$, ${\text{C}}{{\text{l}}_{\text{2}}}$ and ${{\text{O}}_{\text{2}}}$. The gases having molar mass more than that of dry air are heavier than dry air.

 Complete answer:
We know that that dry air is the air which does not contain any moisture or water molecules. The molar mass of dry air is about $28.97{\text{ g/mol}}$.
Moist air is the air which contains moisture or water molecules. Thus, the molar mass of moist air is the sum of molar mass of dry air and the molar mass of water.
The molar mass of moist air is $46.97{\text{ g/mol}}$ which is greater than that of dry air.
Thus, moist air is heavier than dry air. Thus, option (A) is correct.
${\text{S}}{{\text{O}}_{\text{2}}}$ is sulphur dioxide gas. The molar mass of sulphur dioxide gas is $64.06{\text{ g/mol}}$ which is greater than that of dry air.
Thus, ${\text{S}}{{\text{O}}_{\text{2}}}$ is heavier than dry air. Thus, option (B) is correct.
${\text{C}}{{\text{l}}_{\text{2}}}$ is chlorine gas. The molar mass of chlorine gas is $70.90{\text{ g/mol}}$ which is greater than that of dry air.
Thus, ${\text{C}}{{\text{l}}_{\text{2}}}$ is heavier than dry air. Thus, option (C) is correct.
${{\text{O}}_{\text{2}}}$ is oxygen gas. The molar mass of oxygen gas is $31.98{\text{ g/mol}}$ which is greater than that of dry air.
Thus, ${{\text{O}}_{\text{2}}}$ is heavier than dry air. Thus, option (D) is not correct.
Thus, gases heavier than dry air are moist air, ${\text{S}}{{\text{O}}_{\text{2}}}$ and ${\text{C}}{{\text{l}}_{\text{2}}}$.
Thus, the correct options are (A), (B) and (C).

Note: We know that air is a homogeneous mixture of gases like nitrogen, oxygen, argon, carbon dioxide and small quantities of noble gases like helium, neon, krypton along with water vapours, dust and some gases in negligible amounts. When moisture or water vapours are removed from air we get dry air.