Molecular Mass and Formula mass with Examples and Calculations

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.

Example:

Calculate the molecular mass of nitric acid,  HNO₃.

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Solution:

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.

Example:  

Calculate the formula mass of potassium sulphate ( K₂SO₄)

Solution:

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.