What term refers to the relative weight of a gas as compared to the weight of an equal amount of air?

The term that refers to the relative weight of a gas compared to the weight of an equal amount of air is specific gravity. Specific gravity is a dimensionless quantity that is defined as the ratio of the density of a substance to the density of a reference substance—in this case, air.

When discussing gases, specific gravity helps to determine how a particular gas will behave in the atmosphere compared to air. A gas with a specific gravity less than 1 will rise in air, while a gas with a specific gravity greater than 1 will tend to sink. This property is crucial in applications related to ventilation, safety, and the handling of gases.

While gas density is related and pertains to the mass per unit volume of the gas, it does not provide a comparative measure against air. Molecular weight, on the other hand, refers to the mass of a molecule of a specific gas and doesn't provide context about its behavior in relation to air. Relative mass is not a standard term used in this context and could lead to confusion. Specific gravity is the most appropriate term to describe the comparative weight of a gas relative to air.