User Uploaded Image
Air, like any substance, has mass. Each gas and each vapor has it's own unique mass, based on it's atomic number on the periodic table. Since air is a mixture of gasses and vapors, it can get a little tricky to calculate exactly, as the incoming air varies in composition(but, once inside the ball doesn't change in mass). Also, when the soccer ball is filled, the gasses and vapors are compressed, so the volume weighs more than if it was filled to just one atmosphere of pressure- The higher the pressure, the higher the weight. To measure the actual weight difference between empty and filled, you would need a very sensitive scale, capable of measuring to fractions of a gram. So, YES, it is heavier when full. Just not very much.