Rubber doesn't have a single, specific chemical formula. It's a polymer, meaning it's made up of long chains of repeating units. The most common type of rubber is natural rubber, which is made from the sap of rubber trees. Its basic repeating unit is isoprene, which has the formula C5H8.

So, you can't write a simple formula like you would for water (H2O) or salt (NaCl). Instead, you might see something like:

(-CH2-C(CH3)=CH-CH2-)n

This represents the repeating isoprene unit (within the parentheses) and the "n" indicates that this unit repeats many times in a long chain.

There are also many synthetic rubbers, which are made from different monomers and have different chemical formulas. Examples include:

* Neoprene (polychloroprene): (-CH2-CCl=CH-CH2-)n

* Butyl rubber (polyisobutylene): (-CH2-C(CH3)2-CH2-)n

* Silicone rubber: [(CH3)2SiO]n

So, while rubber doesn't have one formula, its basic building blocks are understood, and the specific formula can be determined based on the type of rubber you're considering.