# 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 (Cl_{2}) is 71 amu, of water (H_{2}O) is 18 amu and that of carbon dioxide (CO_{2}) is 44 amu.

**Example: **

Calculate the molecular mass of nitric acid, HNO_{3}.

**Solution:**

Atomic mass of H = 1 amu

Atomic mass of N = 14 amu

Atomic mass of O =16 amu

Molecular formula = HNO_{3}

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 CaCO_{3 }is 100 amu.

**Example: **

Calculate the formula mass of potassium sulphate ( K_{2}SO_{4})

**Solution:**

Atomic mass of K =39 amu

Atomic mass of S = 32 amu

Atomic mass of O= 16 amu

Formula unit= ** **K_{2}SO_{4}

Formula mass of K_{2}SO_{4 }= 2(39)+1(32)+4(16)

= 174 amu. ** **

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