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Atomic and Molecular masses


The average mass per atom of a specified isotopic composition of an element is known as atomic mass. It is simply a mass and thus has the unit of mass (i.e. kg or g). 
From equation (1.8), it follows that,
mass of an atom, A=Ar mau            (1.12)
that is, the mass of an atom (or atomic mass) is equal to the relative atomic mass times the atomic mass unit.


Molecular Mass
The average mass per molecule of a specific isotopic composition of a substance is known as molecular mass. It is simply a mass and thus has the unit of mass (i.e, kg or g). 

From equation (1.9), it follows that, mass of a molecule M= Mr mau (1.13) that is, the mass of a molecule (or molecular mass) is relative molecular mass times the atomic mass unit.

Molar Mass
The average mass per unit amount of substance (applicable to both atoms and molecules) of a specified isotopic composition is known as molar mass. Mathematically, we write it, as

  (1.14)

Since the unit of mass (m) is kg or g and that of amount of substance (n) is mol, it follows that the unit of molar mass is kg mol-1 or g mol-1.

 

Empirical and Molecular Formulae
In chemistry, the elements and compounds are represented by the recommended chemical symbols. The empirical formula of a compound describes the simplest relative number of different types of atoms present in the compound, whereas the molecular formula describes the exact number of different atoms present in it. For example, benzene has a molecular formula C6H6 and its empirical formula will be simply CH. Thus, the molecular formula is an integral multiple of its empirical formula.





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