What is the general formula for a ketone?

Hint: There are various functional groups that are added to organic compounds that impart certain characteristics. Among them are ketones, which are carbonyl groups. Ketones are placed in between any hydrocarbon or organic compound. A carbonyl group has C and O in its formula.

Complete answer:
Functional groups are the extra chemical species or molecules present in any organic compound. The addition of the functional groups, not only changes the name of the organic compound, but also gives it a change in the chemical properties, and the ability to react with certain reagents. The functional groups are responsible for the activity of the organic molecule in various reactions. The functional groups react in the same manner irrespective of the nature or length of the hydrocarbon chain.
Ketone is a type of a functional group used in organic compounds. Ketone consists of a carbonyl group which is a carbon and oxygen. Ketones are added with the general structure of the alkanes in the middle of the chain. The general formula of ketones is, ${{C}_{n}}{{H}_{2n}}O$. Where, n is the number of carbon atoms.
The simplest ketone is of 3 carbon atoms, so its formula will be, ${{C}_{3}}{{H}_{6}}O$, called as propanone, it can also be written as, $C{{H}_{3}}\overset{\overset{O}{\mathop{||}}\,}{\mathop{C}}\,C{{H}_{3}}$, the carbon of the ketone is counted with the carbon of the hydrocarbon.
Hence, the general formula for ketones is ${{C}_{n}}{{H}_{2n}}O$.

Note:
The other functional group that has carbonyl groups is esters, ethers and carboxylic acids. Esters contain the formula RCOOR, where R is any alkyl group, same or different; its group is COO, while ethers have the formula, ROR, where R is any alkyl group, maybe same or different. So, its group is only O. While carboxylic has COOH, with a carbonyl group along with OH.