Final answer:
Option C, which consists of 35 parts dry air and 65 parts water vapor, will weigh the least due to the higher proportion of water vapor, which has a lower density compared to dry air.
Step-by-step explanation:
To determine which of the given options will weigh the least, we must consider the densities of both dry air and water vapor. Dry air typically has a density of approximately 1.20 kg/m³ at 20°C, which is heavier than the density of water vapor (given as 9.40 g/m³, which converts to 0.00940 kg/m³) at the same temperature.
Comparing the options:
- A. 98 parts of dry air and 2 parts of water vapor will result in a mixture closer to the density of air because water vapor makes up a very small portion of the mixture.
- B. 50 parts of dry air and 50 parts of water vapor will result in a mixture with an averaged density that is significantly lower than dry air alone due to the higher proportion of the much less dense water vapor.
- C. 35 parts of dry air and 65 parts of water vapor will have even less dry air and more water vapor, leading to an even lower density mixture.
Therefore, option C, with 35 parts of dry air and 65 parts of water vapor, will weigh the least because it has the highest proportion of the lighter water vapor compared to dry air.