The mass of a substance is independent of the amount of the substance, while density depends on the amount of the substance. the density of a substance is the volume of the substance per mass unit and has units such as l/kg, where volume has the units l and mass has units of kg. the density of a substance is independent of the amount of the substance, while mass depends on the amount of the substance. the density of a substance is the mass of the substance per volume unit and has units such as kg/l, where mass has units of kg and volume has the units l.