CBSE Class 10 Science 2022 · Paper 31(B)/5 · Question 8(a)

(i) A homologous series is a series of carbon compounds that have the same functional group and can be represented by one general formula; successive members differ by a $-{\text{CH}}_2-$ unit ($14$ u in molecular mass), show similar chemical properties and show a gradual change in physical properties. For the compounds with the $-\text{OH}$ functional group (the alcohols) the general formula is ${\text{C}}_n{\text{H}}_{2n+1}\text{OH}$. The third member ($n=3$) is propanol, ${\text{C}}_3{\text{H}}_7\text{OH}$, whose molecular formula is ${\text{C}}_3{\text{H}}_8\text{O}$. (ii) Ethane, ${\text{C}}_2{\text{H}}_6$, has the structure ${\text{H}}_3\text{C}-{\text{CH}}_3$: two carbon atoms joined by a single bond, each also bonded to three hydrogen atoms (drawn in full in the figure). A molecule of ethane has $7$ single bonds: $6$ C–H bonds and $1$ C–C bond.