Homologus series are, carbon compounds grouped together with same functional group in which each member differs from each other by a -CH2 group. Simplest homologus series is alkanes . The first member is methane CH4, than C2H6 and the next will be C3H8 and so on with a difference of CH2 group. One member of homologus series will differ from one another with a molecular weight of 18 units. So the properties depending on molecular weight will be different and chemical properties will be same.