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

The molecular mass of dry air is greater than that of moist air, so the answer is (b) greater than moist air.

Step-by-step solution

Understanding the Concepts

To compare the molecular mass of dry air to that of moist air, we need to understand that dry air primarily consists of nitrogen (N2, with a molar mass of 28.02 g/mol) and oxygen (O2, with a molar mass of 32.00 g/mol), along with small amounts of other gases. Moist air contains all the components of dry air plus water vapor (H2O, with a molar mass of 18.02 g/mol).

Comparing Molecular Masses

Because water vapor has a lower molar mass than the main components of dry air (N2 and O2), the addition of water vapor to dry air (creating moist air) will decrease the average molecular mass of the air, since a smaller proportion of the heavier gas molecules (N2 and O2) will be present in moist air.

Answering the Question

Since the addition of water vapor (which has a lower molar mass) decreases the average molecular mass of air, the molecular mass of dry air is greater than that of moist air.

Key concepts

Molar Mass Comparison

Understanding the concept of molar mass, which is the mass of one mole of a substance measured in grams per mole (g/mol), is crucial when comparing the molecular mass of dry air to that of moist air. It's a fundamental metric in chemistry that provides insight into the composition of compounds and mixtures.

For dry air, its main constituents are nitrogen (N2) and oxygen (O2), which have molar masses of 28.02 g/mol and 32.00 g/mol, respectively. These gases, along with traces of argon, carbon dioxide, neon, and others, contribute to the average molar mass of dry air. When we introduce water vapor into this mixture, as is the case with moist air, we add a component – H2O with a molar mass of 18.02 g/mol – that is significantly lighter than the main gases in dry air.

Let's consider a hypothetical sample of air that is either completely dry or has a known quantity of water vapor. The molar mass of the dry sample would be higher owing to the absence of the lighter water vapor molecules. Conversely, when water vapor is present, it lowers the overall average molar mass of the air because it displaces some of the heavier N2 and O2 molecules. This leads to the conclusion that the molecular mass of dry air is generally greater than that of moist air.

Dry Air Composition

Analyzing the composition of dry air is essential to comprehending why its molecular mass differs from that of moist air. Dry air is a mixture of gases that predominantly contains nitrogen and oxygen, which together make up approximately 99% of the Earth's atmosphere by volume. Specifically, nitrogen constitutes about 78% while oxygen accounts for around 21%.

The remaining 1% consists of argon (0.93%), carbon dioxide (0.04%), and trace amounts of other gases such as neon, helium, methane, krypton, and hydrogen. Since the percentage of each gas contributes to the overall average molar mass of air, calculating this value requires the consideration of the individual molar masses of these gases and their proportional representation in the atmosphere.

Significant Gases in Dry Air

• Nitrogen (N2) - approximately 78%
• Oxygen (O2) - around 21%
• Argon (Ar) - 0.93%
• Carbon Dioxide (CO2) - 0.04%

By knowing the respective proportions and molar masses, we can calculate the average molar mass of dry air, which is crucial for various scientific calculations including those related to atmospheric pressure, pollution dispersion, and even climate modeling.

Moist Air Composition

In contrast to dry air, moist air includes the additional variable of water vapor, which has a significant impact on its properties. The amount of water vapor in the air can vary greatly depending on factors such as temperature, pressure, and relative humidity.

Water vapor is the gaseous state of water and is much lighter than the other major components of air with a molar mass of 18.02 g/mol. It can make up a significant fraction of the air's volume, particularly in warm and humid conditions. The presence of water vapor in the air not only affects the air's molecular mass but also its density and capacity to absorb and retain heat.

Moist air is less dense than dry air at the same temperature and pressure because the lighter water vapor molecules replace some of the heavier nitrogen and oxygen molecules. This distinction is key in meteorology, where moist air dynamics are central to understanding weather patterns and climate. When we discuss the moisture content of the air, it is also important to note that the maximum amount of water vapor that air can hold increases with temperature – meaning warmer air can be more 'moist' than cooler air.

Factors Influencing Moist Air

• Temperature: Warmer air can contain more water vapor.
• Relative Humidity: The amount of water vapor present in the air compared to the amount it can hold at a given temperature.
• Pressure: Impacts the capacity of air to hold water vapor.

The consideration of these factors is essential when assessing the impact of moisture on the average molar mass of air in environmental and engineering calculations.