What is the measured mass of a soccer ball?
It's not correct to talk about "density" of a soccer ball. It's better you say "average density" of a soccer ball. It's clear that any soccer ball consists of different materials such as leather/plastic, rubber, and air. It order to find the average density you have to find the mass of a soccer ball and divide it by it's volume (in the first approximation you neglect the thickness of the shell (leather and rubber coats)).