The term that describes the relative density of a vapor or gas compared to dry air is called:

• A. Density ratio
• B. Vapor pressure
• C. Molar mass
• D. Vapor Density

Answer: (D)

Vapor Density describes how heavy a vapor or gas is relative to dry air. At the same temperature and pressure, a gas’s density scales with its molar mass, so the ratio of the gas’s density to the density of dry air is equal to the gas’s molar mass divided by the air’s molar mass. That ratio is what we call Vapor Density. Since dry air has a molar mass of about 29 g/mol, Vapor Density ≈ M_gas / 29. For example, a gas with molar mass 16 g/mol has a vapor density around 0.55 and tends to rise, while a gas with molar mass 44 g/mol has a vapor density around 1.5 and tends to stay near the ground. Vapor Pressure, in contrast, is about the pressure of a vapor in equilibrium with its liquid and is not the same concept, and Molar mass is a property of the substance, not a relative density to air.