Is the mass of air in two identical rooms the same? One of them is on the first floor, the other is on the ninth. Why?

Not the same, although the difference is very small. This is due to the fact that the mass of air depends on its density, and its density on the ninth floor is slightly lower than on the first. Accordingly, if the air from the room on the ninth floor and on the first one is “weighed” on the same scales, then the weight of the air from the room on the first floor will be greater.