Soap by Whatsapp Cikgu Kwee now! Fatty acids are carboxylic acids that contain 4-28 carbon in the molecule.Soap is the sodium or potassium salt of fatty acid.The general formula of soap is RCOO–Na+ or RCOO–K+, where R is an alkyl group that has long carbon chain.Image below shows the general structure of a soup molecule.Image below shows the example of a soup molecule (Sodium Stearate) Whatsapp Cikgu Kwee now!