Question

Molecular mass of dry air is (a) less than moist air (b) greater than moist air (c) equal to moist air (d) may be greater or less than moist air

Short answer

Question: Comparing the molecular masses, dry air is _____ than moist air. Answer: greater

Step-by-step solution

Determine the Molecular Mass of Dry Air Components

Dry air is composed primarily of nitrogen (N2) with a molecular mass of 28, oxygen (O2) with a molecular mass of 32, and a few other trace gases. The molecular mass of dry air is approximately the weighted average of these components considering their respective percentage concentrations in the air.

Determine the Molecular Mass of Water Vapor

Water vapor (H2O) has a molecular mass of 18, which is lower than both nitrogen and oxygen.

Compare Molecular Masses of Dry Air and Moist Air

Since dry air is composed of gases with molecular masses higher than water vapor, when we introduce water vapor into the air (creating moist air), the overall molecular mass will decrease due to lower molecular mass of water vapor diluting the overall molecular mass of the air mixture.

Select the Correct Option

Based on our analysis, the molecular mass of dry air is greater than that of moist air. Therefore, the correct answer is: (b) greater than moist air

Key concepts

Dry Air Composition

Dry air is a mixture of gases, mostly made up of nitrogen \(N_2\) and oxygen \(O_2\), with trace amounts of other gases. In this mixture, approximately 78% is nitrogen and about 21% is oxygen. The remaining 1% consists of other gases like argon, carbon dioxide, neon, helium, and methane.
When we think of the molecular mass of dry air, we consider the weighted average of these gases. \(N_2\), the most abundant, has a molecular mass of 28, while \(O_2\) has a molecular mass of 32.
This implies that dry air, being made predominantly of nitrogen and oxygen, has a higher molecular mass compared to individual components like helium or methane, creating a baseline for comparing dry air with other mixtures like moist air.

Moist Air

Moist air includes the same components as dry air, but it also contains an additional component: water vapor \(H_2O\). Water vapor is a lighter molecule with a molecular mass of 18. When water vapor mixes with dry air, it changes the make-up of the gas mixture.
It’s interesting to realize that even a small amount of water vapor can significantly affect the overall composition of air. An increase in water vapor content leads to a reduction in the average molecular mass of the air, making moist air different from its dry counterpart. As such, moist air is not simply dry air with added water, but a distinct composition that changes the molecular characteristics of the air itself.

Molecular Mass Comparison

Comparing molecular masses between dry air and moist air reveals an intriguing difference. Dry air primarily composed of \(N_2\) and \(O_2\) carriers relatively high molecular masses. \(N_2\) is 28 and \(O_2\) is 32.
On the other hand, the introduction of water vapor with its lower molecular mass of 18 causes the molecular mass of moist air to decrease. This happens because the heavy gases in dry air are diluted by the lighter water vapor molecules, decreasing the overall average molecular mass.
As a result, despite the intuitive belief that air should be heavier with more substances mixed in, moist air actually ends up having a lower molecular mass compared to dry air.

Water Vapor

Water vapor, the gaseous form of water, plays a significant role in the atmosphere. Unlike liquid water which has a noticeable mass, water vapor is much lighter with a molecular mass of only 18.
This lower molecular mass is key to understanding why moist air is lighter than dry air. Even a small concentration of water vapor can lower the overall molecular mass when mixed into dry air. This property of water vapor contributes to weather patterns and humidity levels across the globe.
In essence, water vapor is a silent influencer of the air we breathe, impacting everything from air density to climate variations by altering the molecular makeup of our atmosphere.