This question was not so popular with the candidates.  Only few candidates attempted this question.  In (a), majority of the candidates do not have good knowledge of hybridization.  In (b) some candidates could not explain why diamond is hard and graphite soft.  In (c), a fair attempt was made at the calculation of the molecular mass of the choroalkane and the drawing of its structure.
The expected answers include:
4.

(a)        (i)         Hybridization is defined as mixing of different atomic orbitals
to produce identical new orbitals (of the same shape and energy).
                                                                                                                       
                        OR
Hybridization is the mixing of two or more orbitals to give new sets of two or
more orbitals which are exactly equivalent.

            (ii)        sp3  and sp2
(iii)       I. s – orbital                 -           spherical 
                        II. p – orbital               -           dumb-bell/pear shape/ figure eight

(b)        In diamond each carbon atom is sp3 hybridized while in graphite each carbon atom is sp2 hybridized. Diamond has a giant covalent structure with a network of strong covalent bonds holding each atom tightly into crystals.

            In graphite, each carbon atom is covalently linked to three neighbouring atoms
in the same plane forming layers of carbon atoms held together by weak
van der Waals forces.                                                                         
(c)        (i)         R – Cl     Ag+    AgCl   
                        1 mole             1 mole
                        M(AgCl) = 108 + 35.5
    = 143.5gmol-1         
                        Moles of AgCl =  mass          
                                                     Molar mass
                                                  
                                                 = 1.280 x 10-3 moles  
I mole of RCl   = 1 mole of AgCl      
\mole of RCl  = 1.280 x 10-3 moles
                          Mass of RCl   = 0.0826 g
                          Molar mass     =    0.0826     
                                                      1.280 x 108 
                                                                         =  64.52 gmol-1     
 
ALTERNATIVE   (A)

            R – Cl     Ag+    AgCl   
            M(AgCl)  =  108 + 35.4  =  143.5 gmol -1
            Moles of AgCl   =    mass     =  0.1837  
                                         Molar mass   143.5
                                                            = 1.280 x 10-3
            143.5 g of AgCl    ≡ 35.5 g of Cl
             0.1837 g of AgCl ≡   35.5  x 0.1837    
                                                143.5
                                           =  0.0454 g
            Hence 0.0454 g of Cl  = 0.0826 g of RCl
                           35.5 g of Cl  = 0.0826  x 35.5 
                                                   0.0454
                                                =  64.52 gmol-1     

            ALTERNATIVE   B

            R – Cl     Ag+    AgCl
            1 mole              1 mole
            M(AgCl)  =  108 + 35.5    =   143 . 5
            0.1837 g of AgCl produced from 0.0826 g of RCl 
            ...         143.5 g of AgCl will be produced from:
                                                0.082   x 143.5  
                                                0.1837
                                          =    64.5 gmol-1     
           
            (ii)        Since the substance is an alkyl chloride/ chloroalkane
                        CnH2n+1Cl = 64.5      
                        =  12n +2n + 1 +35.5 = 64.5
                        14n = 64.5 – 36.5
                        = 28                
                               n =  = 2             
                        Molecular formulae = C2H5Cl

                                    H         H
                                                           
            (iii)       H         C         C         Cl       

                                    H         H

                        Chloroethane