What is the mass of 5.0 cm^3 piece of copper having a density of 8.96 g/cm^3

To solve this problem, multiply 250 g by the latter. It's the same as dividing the mass by the density, but that's really not important here. As long as the units cancel out to a volume unit, everything's fine. 250 g x (1 cm3 / 8.9 g) = 28 cm 3