How do you calculate the mass and volume of an irregular solid object and calculate the density of that object?

Answer 1

You can use a scale for the mass.
then submerge the mass in a beaker of water and see what volume of water it displaces (how high the water rises if in a calibrated beaker). The volume of the solid is the volume of the water displaced when it is fully submerged.
By the way, if the object floats, you do not need the scale. It displaces its mass in water while floating, measure that volume of water displaced and mutiply by the density of water to get mass.
Then submerge it fully to get the volume.
density = mass/volume

Answer 2

Oh, the density of fresh water is about 1 gram/cm^3 or 1000 kg/m^3