Question

Which of the following gases is/are heavier than dry air?
A. Moist air
B. Oxygen
C. Moist nitrogen
D. Hydrogen sulphide

Answer 1

Knowing the exact molar mass of each of the gases will surely help. But there is another way to approximately guess the weights of these gases if you are clear at the fundamentals.

Complete answer:
Air is the homogenous mixture of nitrogen, oxygen, argon, carbon dioxide, very small amounts of noble gases such as helium, neon, krypton, along with water vapour, dust particles and other gases which are negligible when compared to the above mentioned ones.
-Let us first talk about dry air. As the name suggests, this air is devoid of moisture, in other words it does not contain water molecules. Water molecule has a molar mass weight of 18 $g/mol$ which means it will add extra weight to dry air. So that implies moist air is heavier than dry air.
-The molar mass of dry air is about 28.97 $g/mol$approximately. This far outweighs the molar mass of oxygen which is about 16 $g/mol$. Even if we do not remember the weight of dry air we can still come to the same conclusion in another way. As air is a homogenous mixture of multiple gases including oxygen, therefore its weight would never be less than that of oxygen in itself. So oxygen is lighter than dry air.
-Now looking at “moist nitrogen” we can derive our solutions similarly. The dry nitrogen has a molar mass of 14 $g/mol$. Moist nitrogen, as its name suggests is the addition of water molecules to nitrogen gas. But it still falls far behind the molar mass of dry air. Therefore moist nitrogen is lighter than dry air.
-The last option is hydrogen sulphide (${{H}_{2}}S$). It has two molecules of hydrogen and one molecule of oxygen making its molar mass to be 34 $g/mol$. It is very clear that the molar mass of dry air is less than hydrogen sulphide.
We can conclude from the above discussion that moist air and hydrogen sulphide are heavier than dry air.

Note:
Molar mass should not be confused with molecular weight. While the numerical value remains the same the units change significantly. The weight of molar mass is in grams per mole while that of molecular mass is in atomic mass units. Molar mass is the molecular mass of an entity (which may be an atom, ion or molecule) multiplied by the Avogadro’s number.