Which mixture weighs the least for the same volume of gas?

Which mixture weighs the least for the same volume of gas?

• A. 35 parts dry air and 65 parts water vapor
• B. 98 parts dry air and 2 parts water vapor
• C. 50 parts dry air and 50 parts water vapor
• D. 70 parts dry air and 30 parts water vapor

Answer: (A)

Gas mixtures weigh less per unit volume when their average molecular weight is lower. Water vapor has a much lighter molecular weight than the main components of dry air, so increasing the fraction of water vapor lowers the mixture’s overall density. Since water vapor is about 18 g/mol while dry air averages around 29 g/mol, the mixture with the highest proportion of water vapor will have the smallest average molar mass and thus the least weight for the same volume. Among the options, the one with 65% water vapor is the lightest for the same volume. The others have progressively less water vapor and therefore a higher density, meaning more weight per the same volume.

Gas mixtures weigh less per unit volume when their average molecular weight is lower. Water vapor has a much lighter molecular weight than the main components of dry air, so increasing the fraction of water vapor lowers the mixture’s overall density. Since water vapor is about 18 g/mol while dry air averages around 29 g/mol, the mixture with the highest proportion of water vapor will have the smallest average molar mass and thus the least weight for the same volume. Among the options, the one with 65% water vapor is the lightest for the same volume. The others have progressively less water vapor and therefore a higher density, meaning more weight per the same volume.