What Is Formula Mass? How to Calculate Formula Mass of a Substance?

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The formula mass is the sum of the average atomic masses of all atoms of a molecule in its empirical formula.

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The formula mass is also called 'Molecular Mass'. It is expressed in atomic mass units (amu).

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The formula mass is calculated by multiplying the subscript of each element in the formula by its atomic weight.

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Consider chloroform CHCl3. It has one carbon, one hydrogen, & three chlorine atoms.

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Hence, the sum of the average atomic masses of these atoms gives the average molecular mass of chloroform.

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The average atomic weights of C, H, and Cl atoms are 12.01, 1.008, and 35.45 amu, respectively.

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So, the molecular mass of CHCl3 is (112.01) + (11.008) + (3*35.45) = 119.37 amu.

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The formula mass for ionic compounds is the sum of all the atoms' average atomic masses in the compound’s formula.

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NaCl is an example of an ionic compound. Its formula mass is 22.99(atomic mass of Na)+35.45(atomic mass Cl)=58.44 amu.

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