One group, C=O, and four families. Compare them, climb the oxidation ladder, build an ester from an acid and an alcohol, and work out the pH of a weak acid at any strength you like.
An aldehyde keeps a hydrogen on the carbonyl carbon, and that hydrogen is what lets it oxidize further.
Oxidizes easily to the matching carboxylic acid.
Only a small share of the molecules hand over a proton, which is what makes these weak acids.