Vapor Density is defined as

• A. The density of a liquid relative to water.
• B. The rate at which a vapor diffuses in air.
• C. The density of a vapor or gas in comparison to dry air.
• D. The total mass of vapor per unit volume.

Answer: (C)

Vapor density measures how heavy a vapor is compared with the surrounding air. It’s defined as the density of the vapor divided by the density of dry air at the same temperature and pressure, giving a dimensionless number. If the vapor density is less than 1, the gas is lighter than air and will tend to rise; if greater than 1, it will tend to collect in low areas.

This is why the correct choice describes the density of a vapor or gas in comparison to dry air. The other ideas don’t fit: density relative to water applies to liquids, not gases; diffusion rate describes how quickly a substance spreads through air, not how heavy it is compared to air; and total mass per unit volume is the actual density of the gas, which depends on temperature and pressure but isn’t expressed as a ratio to air.