There is no single chemical formula for a glass rod because glass is not a pure compound but rather a mixture of various oxides. The composition of glass can vary greatly depending on its intended use.

Here's a breakdown:

* Common glass (like window glass) is primarily made of silica (SiO2), with other oxides like sodium oxide (Na2O) and calcium oxide (CaO) added for specific properties.

* Other types of glass might contain different oxides like lead oxide (PbO), boron oxide (B2O3), or potassium oxide (K2O).

Therefore, it's more accurate to describe the composition of a glass rod by listing its major components, rather than providing a single chemical formula.

For example, a common soda-lime glass rod might have a composition like this:

* SiO2 (silica): 72%

* Na2O (sodium oxide): 15%

* CaO (calcium oxide): 10%

* Other oxides: 3%

Keep in mind that this is just a general example, and the specific composition can vary depending on the specific type of glass.