The general formula of both aldehydes and ketones is:

Let us take an aldehyde, ethanal. The formula of ethanal is 퐶퐻3퐶퐻푂, so the number of carbon atoms is 2 and the number of hydrogen atoms is 4, and the number of oxygen atom is one. Now let us take a ketone, acetone. The formula of acetone is퐶퐻3퐶푂퐶퐻3, so the number of carbon atoms is 3, the number of hydrogen atoms is 6, and the number of oxygen atom is one. So, from this we can conclude the general formula of aldehydes and ketones as 퐶푛퐻2푛푂.