homologous series 係指一種compound可以俾同一個general formula represent  

咁ester 同alkanoic acid係咪同一個homologous series 因為佢地 general formula 都係CnH2nO2
但佢地好似唔同group ?

In chemistry, a homologous series is a series of organic compounds with a similar general formula , possessing similar chemical properties due to the presence of the same functional group, and shows a gradation in physical properties as a result of increase in molecular size and mass.

Thus the esters and the alkanoic acids don't belong to the same homlogous series because they don't possess the same functional group.

A LEVEL ?