Chief Editor
The term molecular formula may not be used to represent every compound; rather certain compounds are represented by their formula units. Why? 
Difficulty: Medium

Tonic compounds are represented by formula unit (formula mass) and covalent compounds are represented by their molecular mass Sodium chloride is an ionic compound and it contains sodium ions (Na+) and chloride ions (CI-) present in it in a simple ratio of 1:1. Thus NaCl may also be called its formula unit in comparison, covalent compounds like Sulphur dioxide exists in form of molecules having a molecular formula SO2 Similarly hydrogen sulphites and water have molecular formulas H2S and H2O respectively.