You know that each carbon atom can take as much as 4 bonds with other elements. A hydrogen can only take one bond, while an oxygen can take 2. Thus, the chemical formula of H₂CO may be written as H₂C=O which is acetaldehyde. Acetaldehyde is an aldehyde. It has a higher vapor pressure because aldehydes have much stronger bonds than hydrogen bonds in alcohols. Hence, it would need a lot energy, thus higher vapor pressure, to break the bonds of an aldehyde than it would for an alcohol.