Formula mass of a substance is the sum of the average atomic masses of each atom represented in the chemical formula.

It is expressed in atomic mass units.

Formula mass (M) is used for a substance made up of molecules.

**Formula to calculate formula mass.**

**Example:**

Calculate the formula mass of Carbon (i) Oxide.

Carbon (i) Oxide is a compound made of two element; carbon and oxygen.

The atomic mass of carbon is **12** while the atomic mass of oxygen is **16**, therefore the formula mass of CO is:

**12 + 16 = 28**

Therefore, the formula mass of CO is 28 amu.