The general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}\mathrm{O}$ is true for

The general formula of aldehyde (Alkanol) is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}\mathrm{O}$ 
The general formula of Ketones (Alkanones) is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}\mathrm{O}$ 
The general formula of alcohols (Alkanol) having one double bond also ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}\mathrm{O}$.
i.e Alkenol.