Which term is used to determine vapor density?

• A. Vapor Pressure
• B. Molecular Weight
• C. Specific Gravity
• D. Boiling Point

Answer: (B)

Vapor density is determined by the molecular weight of the gas. At the same temperature and pressure, a gas’s density scales with its molar mass, so heavier molecules create a heavier vapor. Knowing the molecular weight lets you compare to air (which has a mean molar weight of about 29 g/mol) to judge whether the vapor will be heavier or lighter than air and thus how it will behave in the environment. Vapor pressure, boiling point, and related density concepts don’t directly tell you how heavy the vapor is, so they don’t serve to determine vapor density the way molecular weight does.