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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

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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.

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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.