In the equation Yg = ?g / ?air, which statement describes ?air?

• A. The density of the gas
• B. The molar mass of air
• C. The temperature of the air
• D. The density of dry air

Answer: (D)

The idea being tested is how gas specific gravity is defined: it’s the ratio of the gas density to the density of the reference air, using both at the same temperature and pressure. Because of that, the denominator must be the density of dry air. This provides a dimensionless comparison of how dense the gas is relative to air. The other options describe properties that aren’t the reference density used in this ratio—the density of the gas would be the numerator, while molar mass or temperature of air aren’t what the denominator represents (though they influence density under given conditions).