Final answer:
The mass of air in a flask can be neglected when measuring the mass of water due to the significant mass difference between water and air. In contrast, when measuring the mass of carbon dioxide in the flask, the mass of air cannot be neglected because the densities of CO2 and air are closer to one another, making the air's mass more significant relative to CO2.
Step-by-step explanation:
When considering measurements in chemistry experiments, the mass of air within a flask is often negligible when calculating the mass of water due to water's significantly greater density and mass in comparison to air. However, this is not the case when measuring the mass of carbon dioxide (CO2) because the density of CO2 is much closer to the density of air. The mass of CO2 would be more significantly affected by the presence of air. Since CO2 gas is being measured rather than a liquid, the volume that air occupies and its respective mass can't be neglected due to the comparable weight relative to the mass of CO2.
The determination of the gas mass can be made by using density calculations. This means that for substances with very small mass differences compared to the vessel in which they are contained, like gases, we must account for the mass of air to ensure accurate measurements of mass. However, this is less critical with much heavier substances like liquids that far outweigh the mass of air present in the same container.