Why do compounds have same emperical and moleculer formula?

Answer 1

Empirical Formula

All compounds are formed from different atoms of different elements through chemical bonding. Especially in organic chemistry, there are billions of different compounds which has made up from hydrogen and carbon.

As an example let us take ethene and propene, which are two alkenes.

The prior has 2 carbon atoms and 4 hydrogen atoms, and the latter has 3 carbon atoms and 6 hydrogen atoms.

In both of these molecules, the simplest ratio between C:H is 1:2, hence both of these compounds have the empirical formula of CH2.

Molecular FormulaThe molecular formula is given by the actual ratio of the atoms which are in the molecule. For example, ethanol and diethyl ether both have the same molecular formula of C2H6O (in the practice we write these with different notations though). Compounds with the same molecular formula are known as isomers. Constitutional isomers, Stereo-isomers and Conformational isomers are three main types of isomers.

Answer 2

Compounds may have the same empirical and molecular formulas if their molecular structures are simple and do not contain subunits or different isomers. In such cases, the simplest ratio of their elements is the same as the actual ratio of their atoms in a molecule. This leads to the empirical and molecular formulas being identical.