Molecular Mass and Formula mass with Examples and Calculations

what is the difference between molecular mass and formula mass ?

The sum of atomic masses of all atoms present in one molecule of a molecular substance,is its molecular mass. For example, molecular mass of chlorine (Cl2) is 71 amu, of water (H2O) is 18 amu and that of carbon dioxide (CO2) is 44 amu.


Calculate the molecular mass of nitric acid,  HNO3.



Atomic mass of H  =  1 amu

Atomic mass of N  = 14 amu

Atomic mass of O  =16 amu

Molecular formula = HNO3

Molecular mass = 1 ( At. Mass of H)+1(At.mass of N) + 3(At.mass of O)

= 1                              + 14                       + 3(16)

= 1                                 +14                        +48

= 63 amu

Ionic compounds that form three-dimensional solid crystals, are represented by their formula units, formula mass. In such cases  is the sum of atomic masses of all the atoms present in one formula unit of a substance. For example, formula mass of sodium chloride is 58.5 amu and that of CaCO3 is 100 amu.


Calculate the formula mass of potassium sulphate ( K2SO4)


Atomic mass of K =39 amu

Atomic mass of S = 32 amu

Atomic mass of O= 16 amu

Formula unit=    K2SO4

Formula mass of K2SO4 = 2(39)+1(32)+4(16)

= 174 amu.                       

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