Question #470b1

Acetic acid belongs to the family of carboxylic acids. Formic acid (or methanoic) is the simplest, and its formula is $H C \left(= O\right) O H$; the next member of the homologous series is ${H}_{3} C - C \left(= O\right) O H$, ethanoic or acetic acid, then propanoic (propionic acid) acid, butanoic acid (butyric acid), and so on. The acidity (the ${K}_{a}$ values) of these acids decrease down the series (as the carbon chain gets longer). Why?