General formula for Carbohydrates?

Carbohydrates have one carbon to every water molecule.

For monosaccharides the (C H_2 O)n , where n=3 gives a triose sugar

C3H6O3

http://en.wikipedia.org/wiki/Aldose

Hexoses are C6H12O6

http://en.wikipedia.org/wiki/Hexose

This is true even in very long polysaccharides

You're very inquisitive! I like that.

There can only be a certain limit of the ratio between carbons to hydrogens/oxygens, since hydrogen and oxygen atoms depend on how many carbon atoms (the carbon rings are like the backbone of the molecule) and a carbon atom can only have up to four bonds.

I think the varying number of oxygens in the ratios they give you is because most often there isn't a solitary oxygen bound to a carbon, it's usually just one hydrogen atom or a hydroxyl (-OH) group. So for the most part you know there will be hydrogen attached to a carbon in the backbone but not necessarily oxygen.

I'm not 100% sure about all that; it's only my interpretation. I hope this helps.

A carbohydrate is an organic compound with the empirical formula Cm(H2O)n

(where m could be different from n); that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 (as in water). Carbohydrates can be viewed as hydrates of carbon, hence their name. Structurally however, it is more accurate to view them as polyhydroxy aldehydes and ketones.