Chemistry

# 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, the molecular mass of chlorine (Cl2) is 71 AMU, water (H2O) is 18 AMU and carbon dioxide (CO2) is 44 AMU.

Example:

Calculate the molecular mass of nitric acid,  HNO3.

Solution:

The atomic mass of H  =  1 amu

The atomic mass of N  = 14 amu

The 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, the formula mass of sodium chloride is 58.5 AMU and that of CaCO3 is 100 AMU.

Example:

Calculate the formula mass of potassium sulfate ( K2SO4)

Solution:

The atomic mass of K =39 amu

The atomic mass of S = 32 amu

The 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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