An air-filled bladder has more mass than an empty bladder. Why?
Because there is more air. The air has pasta and takes up space. But in the case of the bladder, the mass difference is very small and can only be measured on very sensitive scales.
The mass difference is small because the air density is relatively small - much smaller, for example, than the water density.
Now consider this situation: you smell a tasty cake or other food coming from the kitchen. In fact, you are feeling the effect of gases coming out of the food that stimulated certain parts of your nose. This is due to a property of air and all gases: they tend to spread, filling all available space. Therefore, the gases that come out of the food spread through the house.
Compare the gases with the liquids: when you pour some water into a bottle without filling it, the water will settle to the bottom. It does not occupy the entire volume of the bottle. But on the other hand, whatever air is inside a bottle, it will be taking up the entire space of the bottle. Air, and gases in general, occupy the entire volume of the container where they are. It is the property of expandability.
When we blow a birthday bladder, filling it well, we find that the balloon wall is well stretched. This is due to another property of air and gases: they exert pressure against the wall of the container they occupy.
The pressure exerted by air on the earth's surface is called atmospheric pressure. It gets its name because the atmosphere is the layer of air that surrounds the planet.