Vapor density is defined as:

Vapor density is defined as:

• A. Weight of a vapor relative to water.
• B. The density of the liquid from which the vapor comes.
• C. Weight of pure vapor or gas compared to the weight of an equal volume of dry air at the same temperature and pressure.
• D. The mass of gas per mole.

Answer: (C)

Vapor density is a comparison of how heavy a vapor is in air. It tells you the weight of a given volume of the vapor relative to the weight of an equal volume of dry air at the same temperature and pressure. This relative measure is what helps you predict where a gas will travel in the environment: heavier-than-air vapors tend to linger in low areas, while lighter-than-air vapors rise and disperse. The definition described—weight of a pure vapor or gas compared to the weight of an equal volume of dry air under the same conditions—fits this concept exactly, making it the best choice. It’s not about the liquid’s density, nor about density relative to water, nor simply the molecular weight by itself. Those describe other properties, whereas vapor density specifically compares to air to indicate how the gas will behave in the atmosphere. In practice, you can think of vapor density as the gas’s density relative to air (often approximated via its molecular weight compared to that of air, about 28.97 g/mol), with higher values indicating a tendency to settle lower and lower values indicating it will rise.

Vapor density is a comparison of how heavy a vapor is in air. It tells you the weight of a given volume of the vapor relative to the weight of an equal volume of dry air at the same temperature and pressure. This relative measure is what helps you predict where a gas will travel in the environment: heavier-than-air vapors tend to linger in low areas, while lighter-than-air vapors rise and disperse.

The definition described—weight of a pure vapor or gas compared to the weight of an equal volume of dry air under the same conditions—fits this concept exactly, making it the best choice. It’s not about the liquid’s density, nor about density relative to water, nor simply the molecular weight by itself. Those describe other properties, whereas vapor density specifically compares to air to indicate how the gas will behave in the atmosphere. In practice, you can think of vapor density as the gas’s density relative to air (often approximated via its molecular weight compared to that of air, about 28.97 g/mol), with higher values indicating a tendency to settle lower and lower values indicating it will rise.