Which term is used to describe the ratio of the average molecular weight of a gas or vapor to the average molecular weight of air?

Which term is used to describe the ratio of the average molecular weight of a gas or vapor to the average molecular weight of air?

• A. Specific Gravity
• B. Molar Ratio
• C. Gas Density Ratio
• D. Specific Gravity of Air

Answer: (D)

The main idea is that a gas’s tendency to rise or sink is described by a dimensionless ratio called the specific gravity relative to air. This is defined as the gas’s average molecular weight divided by the average molecular weight of air (SG = MW_gas / MW_air). Because density at the same temperature and pressure is proportional to molecular weight, this ratio tells you whether the gas is heavier or lighter than air: SG greater than 1 means heavier than air and tends to collect in low areas; SG less than 1 means lighter and tends to rise. Air has an average molecular weight of about 29 g/mol, so you can think of SG as MW_gas divided by 29. For example, helium gives SG around 0.14 (much lighter than air), while sulfur hexafluoride gives SG around 5 (much heavier). Thus, the term describing this ratio is the specific gravity of the gas relative to air (often referred to as the gas’s specific gravity with air as the reference).

The main idea is that a gas’s tendency to rise or sink is described by a dimensionless ratio called the specific gravity relative to air. This is defined as the gas’s average molecular weight divided by the average molecular weight of air (SG = MW_gas / MW_air). Because density at the same temperature and pressure is proportional to molecular weight, this ratio tells you whether the gas is heavier or lighter than air: SG greater than 1 means heavier than air and tends to collect in low areas; SG less than 1 means lighter and tends to rise.

Air has an average molecular weight of about 29 g/mol, so you can think of SG as MW_gas divided by 29. For example, helium gives SG around 0.14 (much lighter than air), while sulfur hexafluoride gives SG around 5 (much heavier).

Thus, the term describing this ratio is the specific gravity of the gas relative to air (often referred to as the gas’s specific gravity with air as the reference).