We know that some materials are light while some are very heavy. For instance, a cube of polythene will weigh a lot less than the same size cube of steel.
We can systematically compare amount of matter in different materials by defining a property called density (r).
Density = mass per unit volume
M is the mass of an object, and V is its volume.
Units of ρ: kgm-3.
Tip: From the knowledge of density and molar mass of a solid or liquid, it is possible to estimate the average volume occupied by each molecule in a substance (Vmolecule) by using the following formula.
Where, Mmole its molar mass, and
A = Avagadro's number = 6.023 x 1023.
Approximate molecular spacing (i.e. distance D between adjacent molecules) in the material is given by the cube root of the volume of the molecule.
D = (Vmolecule)1/3.
Additional Definitions: Molar mass of a material = mass of one mole of the material.
Mole of a material = amount of material containing 6.023 x 1023 molecules.