In order to determine the mass of the piece of copper with only knowing it's density, it must have it's volume measured. In order to do this, the piece of copper could be placed in a container filled to the brim with a spout and a graduated cylinder below the spout. Then the piece of copper can be placed into the container without any fingers in the water. Then water will displace into the cylinder, the measured volume is the volume of the piece of copper, because of Archimede's principle, and we assume that the water is 1.0 g/mL or 1 g/cm^3. Then multiply the density with the volume to get the mass of the piece of copper.