What is the chemical formula of Vitamin C?

Vitamin A is technically a group of chemicals with a similar function, that is, they can be converted into Retinal, an aldehyde which absorbs light, essential for colour vision.

The most common form of Vitamin A is retinol, C20H30O.
In the active form, it goes by the somewhat unwieldy name (2E,4E,6E,8E)-3,7-dimethyl-9-(2,6,6-trimethylcyclohexen-1-yl)nona-2,4,7,8-tetraenal. For reasons I hope are fairly obvious, it's more commonly called retinal. The molecular formula is C20H28O. (The only difference between retinol and retinal is that one is an alcohol and the other is the aldehyde form of that alcohol.)