Here's why:

* Complex mixture: Coal is a complex mixture of various organic compounds, primarily carbon, hydrogen, and oxygen, along with smaller amounts of nitrogen, sulfur, and trace elements.

* Variable composition: The exact composition of coal varies greatly depending on its geological origin, age, and rank (ranging from lignite to anthracite).

* No distinct molecules: The organic compounds in coal are not arranged in distinct, repeating molecular structures like a typical chemical compound.

Instead of a molecular formula, coal is often characterized by:

* Elemental analysis: This determines the percentages of carbon, hydrogen, oxygen, nitrogen, and sulfur.

* Proximate analysis: This measures moisture, volatile matter, fixed carbon, and ash content.

* Ultimate analysis: This provides a more detailed breakdown of the chemical elements present.

So, while you can't write a single molecular formula for coal, you can describe its composition using these analytical techniques.