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 Where, 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.