Difference between Formula unit and Formula mass

What is Formula unit?

In Chemistry, the simplest ratio between cations & anions in an ionic compound is called formula unit.


Simplest unit whose repetition builds crystal is called formula unit. Usually for ionic compounds their empirical formula is their formula unit while for covalent compound their molecular formula is their formula unit.

For Example :

Common salt consist of Na+ and Cl- ions .For every Na+ ion there is one Cl- ion. Therefore, the formula unit of common salt is NaCl .

What is Formula mass?

Sum of relative atomic masses of atoms in a formula unit of a compound is called formula mass.

For example :

Formula mass Calculation of Compounds:

(i) formula mass of MgSO4 = 24 + 32 + 16 x 4 = 24 + 32 + 64 = 120

(ii) formula mass of C3H6O = 12 x 3 + 1 x 6 + 16 = 36 + 6 + 16 = 58

(iii) formula mass of C3H8 = 12 x 3 + 1 x 8 = 36 + 8 = 44

(iv) formula mass of C2H5OH = 12 x 2 + 1 x 6 + 16 = 24 + 6 + 16 = 46

(v) formula mass of Al2O3 = 27 x 2 + 16 x 3 = 54 + 48 = 102

(vi) formula mass of K2Cr2O7 = 39 x 2 + 52 x 2 + 16 x 7 = 78 + 104 + 112 = 294

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